signalfd: fix error return code

If anon_inode_getfile() fails, return appropriate error code. This looks
like a single typo: the similar code changes in timerfd and userfaultfd
are okay.

Found by Linux Verification Center (linuxtesting.org).

Fixes: fbe38120 ("signalfd: convert to ->read_iter()")
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Link: https://lore.kernel.org/r/20240520090819.76342-1-pchelkin@ispras.ru


Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>