nsfs: use cleanup guard

Ensure that rcu read lock is given up before returning.

Link: https://lore.kernel.org/r/20240716-elixier-fliesen-1ab342151a61@brauner
Fixes: ca567df7

 ("nsfs: add pid translation ioctls")
Reported-by:  <syzbot+a3e82ae343b26b4d2335@syzkaller.appspotmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>