bpf: Call the missed kfree() when there is no special field in btf

Call the missed kfree() in btf_parse_struct_metas() when there is no
special field in btf, otherwise will get the following kmemleak report:

unreferenced object 0xffff888101033620 (size 8):
  comm "test_progs", pid 604, jiffies 4295127011
  ......
  backtrace (crc e77dc444):
    [<00000000186f90f3>] kmemleak_alloc+0x4b/0x80
    [<00000000ac8e9c4d>] __kmalloc_cache_noprof+0x2a1/0x310
    [<00000000d99d68d6>] btf_new_fd+0x72d/0xe90
    [<00000000f010b7f8>] __sys_bpf+0xec3/0x2410
    [<00000000e077ed6f>] __x64_sys_bpf+0x1f/0x30
    [<00000000a12f9e55>] x64_sys_call+0x199/0x9f0
    [<00000000f3029ea6>] do_syscall_64+0x3b/0xc0
    [<000000005640913a>] entry_SYSCALL_64_after_hwframe+0x4b/0x53

Fixes: 7a851ec ("bpf: Search for kptrs in prog BTF structs")
Signed-off-by: Hou Tao <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

kernel/bpf/btf.c

@@ -5572,8 +5572,10 @@ btf_parse_struct_metas(struct bpf_verifier_log *log, struct btf *btf)
 		aof->ids[aof->cnt++] = i;
 	}
 
-	if (!aof->cnt)
+	if (!aof->cnt) {
+		kfree(aof);
 		return NULL;
+	}
 	sort(&aof->ids, aof->cnt, sizeof(aof->ids[0]), btf_id_cmp_func, NULL);
 
 	for (i = 1; i < n; i++) {