io_uring/msg_ring: check file type before putting

If we're invoked with a fixed file, follow the normal rules of not
calling io_fput_file(). Fixed files are permanently registered to the
ring, and do not need putting separately.

Cc: stable@vger.kernel.org
Fixes: aa184e86 ("io_uring: don't attempt to IOPOLL for MSG_RING requests")
Signed-off-by: Jens Axboe <axboe@kernel.dk>