ovl: call posix_acl_release() after error checking

The current placement of posix_acl_release() in ovl_set_or_remove_acl()
means it can be called on an error pointer instead of actual acls.
Fix this by moving the posix_acl_release() call after the error handling.

Fixes: 0e641857 ("ovl: implement set acl method") # mainline only
Reported-by:  <syzbot+3f6ef1c4586bb6fd1f61@syzkaller.appspotmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>