apparmor: fix bind mounts aborting with -ENOMEM

With commit df323337e507 ("apparmor: Use a memory pool instead per-CPU
caches, 2019-05-03"), AppArmor code was converted to use memory pools. In
that conversion, a bug snuck into the code that polices bind mounts that
causes all bind mounts to fail with -ENOMEM, as we erroneously error out
if `aa_get_buffer` returns a pointer instead of erroring out when it
does _not_ return a valid pointer.

Fix the issue by correctly checking for valid pointers returned by
`aa_get_buffer` to fix bind mounts with AppArmor.

Fixes: df323337e507 ("apparmor: Use a memory pool instead per-CPU caches")
Signed-off-by: Patrick Steinhardt <[e-mail]>
Signed-off-by: John Johansen <[e-mail]>

security/apparmor/mount.c

···
 	buffer = aa_get_buffer(false);
 	old_buffer = aa_get_buffer(false);
 	error = -ENOMEM;
-	if (!buffer || old_buffer)
+	if (!buffer || !old_buffer)
 		goto out;
 
 	error = fn_for_each_confined(label, profile,