io_uring: unify getting ctx from passed in file descriptor

io_uring: unify getting ctx from passed in file descriptor

io_uring_enter() and io_uring_register() end up having duplicated code
for getting a ctx from a passed in file descriptor, for either a
registered ring descriptor or a normal file descriptor. Move the
io_uring_register_get_file() into io_uring.c and name it a bit more
generically, and use it from both callsites rather than have that logic
and handling duplicated.

Signed-off-by: Jens Axboe <axboe@kernel.dk>