powerpc/rtas_flash: allow user copy to flash block cache objects

With hardened usercopy enabled (CONFIG_HARDENED_USERCOPY=y), using the
/proc/powerpc/rtas/firmware_update interface to prepare a system
firmware update yields a BUG():

kernel BUG at mm/usercopy.c:102!
Oops: Exception in kernel mode, sig: 5 [#1]
LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
Modules linked in:
CPU: 0 PID: 2232 Comm: dd Not tainted 6.5.0-rc3+ #2
Hardware name: IBM,8408-E8E POWER8E (raw) 0x4b0201 0xf000004 of:IBM,FW860.50 (SV860_146) hv:phyp pSeries
NIP:  c0000000005991d0 LR: c0000000005991cc CTR: 0000000000000000
REGS: c0000000148c76a0 TRAP: 0700   Not tainted  (6.5.0-rc3+)
MSR:  8000000000029033 <SF,EE,ME,IR,DR,RI,LE>  CR: 24002242  XER: 0000000c
CFAR: c0000000001fbd34 IRQMASK: 0
[ ... GPRs omitted ... ]
NIP [c0000000005991d0] usercopy_abort+0xa0/0xb0
LR [c0000000005991cc] usercopy_abort+0x9c/0xb0
Call Trace:
[c0000000148c7940] [c0000000005991cc] usercopy_abort+0x9c/0xb0 (unreliable)
[c0000000148c79b0] [c000000000536814] __check_heap_object+0x1b4/0x1d0
[c0000000148c79f0] [c000000000599080] __check_object_size+0x2d0/0x380
[c0000000148c7a30] [c000000000045ed4] rtas_flash_write+0xe4/0x250
[c0000000148c7a80] [c00000000068a0fc] proc_reg_write+0xfc/0x160
[c0000000148c7ab0] [c0000000005a381c] vfs_write+0xfc/0x4e0
[c0000000148c7b70] [c0000000005a3e10] ksys_write+0x90/0x160
[c0000000148c7bc0] [c00000000002f2c8] system_call_exception+0x178/0x320
[c0000000148c7e50] [c00000000000d520] system_call_common+0x160/0x2c4

arch/powerpc/kernel/rtas_flash.c

@@ -709,9 +709,9 @@ static int __init rtas_flash_init(void)
 	if (!rtas_validate_flash_data.buf)
 		return -ENOMEM;
 
-	flash_block_cache = kmem_cache_create("rtas_flash_cache",
-					      RTAS_BLK_SIZE, RTAS_BLK_SIZE, 0,
-					      NULL);
+	flash_block_cache = kmem_cache_create_usercopy("rtas_flash_cache",
+						       RTAS_BLK_SIZE, RTAS_BLK_SIZE,
+						       0, 0, RTAS_BLK_SIZE, NULL);
 	if (!flash_block_cache) {
 		printk(KERN_ERR "%s: failed to create block cache\n",
 				__func__);