Check that journal was initialized successfully

Otherwise you will next trace:
[  964.775061] The magic number obtained in disk is: [268640275]
[  964.775554] simplefs filesystem of version [1] formatted with a block size of [4096] detected in the device.
[  964.776308] Loading journal devnum: 63
[  964.776724] Journal device is: unknown-block(0,63)
[  964.779494] simplefs superblock is destroyed. Unmount succesful.
[  964.781366] Freeing private data of inode ffff88007babe3a8 (1)
[  964.781903] simplefs is succesfully mounted on [/dev/loop0]
[  964.782393] BUG: unable to handle kernel NULL pointer dereference at 0000000000000069
[  964.783381] IP: [<ffffffff8112f0b3>] mount_fs+0x71/0x148
[  964.783977] PGD 7a545067 PUD 7a514067 PMD 0
[  964.784814] Oops: 0000 [#1] PREEMPT SMP
[  964.785176] Modules linked in: simplefs(PO) loop fuse joydev hid_generic usbhid hid snd_pcm microcode snd_timer snd psmouse soundcore evdev pcspkr serio_raw ohci_pci ohci_hcd ehci_hcd processor parport_pc usbcore button battery ac thermal_sys parport i2c_piix4 usb_common i2c_core ext4 crc16 jbd2 mbcache sg sd_mod sr_mod crc_t10dif cdrom crct10dif_common ata_generic ahci libahci ata_piix libata scsi_mod e1000 [last unloaded: simplefs]
[  964.785176] CPU: 0 PID: 3507 Comm: mount Tainted: P           O 3.14.0-rc6+ #101
[  964.785176] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[  964.785176] task: ffff88007cb8c6d0 ti: ffff88007ad04000 task.ti: ffff88007ad04000
[  964.785176] RIP: 0010:[<ffffffff8112f0b3>]  [<ffffffff8112f0b3>] mount_fs+0x71/0x148
[  964.785176] RSP: 0018:ffff88007ad05e40  EFLAGS: 00010203
[  964.785176] RAX: 0000000000000001 RBX: ffff88007c0fd000 RCX: 0000000000000007
[  964.785176] RDX: 0000000000000001 RSI: 0000000000000046 RDI: 00000000ffffffff
[  964.785176] RBP: ffff88007ad05e70 R08: 0000000000000004 R09: ffff8800000bbca0
[  964.785176] R10: fffffff000007bf4 R11: 0000000000000000 R12: ffff88007a329000
[  964.785176] R13: 0000000000000001 R14: ffffffffa01d2000 R15: 0000000000000000
[  964.785176] FS:  00007f195521f840(0000) GS:ffff88007fc00000(0000) knlGS:0000000000000000
[  964.785176] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  964.785176] CR2: 0000000000000069 CR3: 000000007a540000 CR4: 00000000000006f0
[  964.785176] Stack:
[  964.785176]  ffff88007a1de080 ffff88007a1de080 ffffffffa01d2000 ffff8800377b2220
[  964.785176]  ffff88007c0fd000 ffffffffa01d2000 ffff88007ad05ea8 ffffffff81143628
[  964.785176]  0000000081142d1b ffff88007c0fd000 ffff8800377b2220 0000000000000027
[  964.785176] Call Trace:
[  964.785176]  [<ffffffff81143628>] vfs_kern_mount+0x5f/0xd3
[  964.785176]  [<ffffffff811458e9>] do_mount+0x6f9/0x7f0
[  964.785176]  [<ffffffff81145c01>] SyS_mount+0x80/0xb9
[  964.785176]  [<ffffffff813d2262>] system_call_fastpath+0x16/0x1b
[  964.785176] Code: cf 06 00 85 c0 0f 85 d0 00 00 00 4c 89 ea 48 89 d9 44 89 fe 4c 89 f7 41 ff 56 10 49 89 c5 49 81 fd 00 f0 ff ff 0f 87 b0 00 00 00 <49> 8b 5d 68 48 85 db 75 02 0f 0b 48 83 bb d0 00 00 00 00 75 11
[  964.785176] RIP  [<ffffffff8112f0b3>] mount_fs+0x71/0x148
[  964.785176]  RSP <ffff88007ad05e40>
[  964.785176] CR2: 0000000000000069
[  964.820009] ---[ end trace fc63610f0693bff7 ]---

simple.c

@@ -722,6 +722,10 @@ static int simplefs_load_journal(struct super_block *sb, int devnum)
 	len = SIMPLEFS_MAX_FILESYSTEM_OBJECTS_SUPPORTED;
 
 	journal = jbd2_journal_init_dev(bdev, sb->s_bdev, 1, len, blocksize);
+	if (!journal) {
+		printk(KERN_ERR "Can't load journal\n");
+		return 1;
+	}
 	journal->j_private = sb;
 
 	sfs_sb->journal = journal;