switch io_msg_ring() to CLASS(fd)

Use CLASS(fd) to get the file for sync message ring requests, rather
than open-code the file retrieval dance.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/20241115034902.GP3387508@ZenIV


[axboe: make a more coherent commit message]
Signed-off-by: Jens Axboe <axboe@kernel.dk>