apparmor: fix bind mounts aborting with -ENOMEM

With commit df32333 ("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: df32333 ("apparmor: Use a memory pool instead per-CPU caches")
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: John Johansen <john.johansen@canonical.com>

Author: pks-t

security/apparmor/mount.c

@@ -442,7 +442,7 @@ int aa_bind_mount(struct aa_label *label, const struct path *path,
 	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,