apparmor: Fix return in ns_mkdir_op

apparmor: Fix return in ns_mkdir_op

Return NULL instead of passing to ERR_PTR while error is zero.
  Fixes smatch warning:
    - security/apparmor/apparmorfs.c:1846 ns_mkdir_op() warn:
      passing zero to 'ERR_PTR'

Fixes: 88d5baf69082 ("Change inode_operations.mkdir to return struct dentry *")
Reviewed-by: Ryan Lee <ryan.lee@canonical.com>
Signed-off-by: Hongling Zeng <zenghongling@kylinos.cn>
Signed-off-by: John Johansen <john.johansen@canonical.com>