io_uring/register: provide helper to get io_ring_ctx from 'fd'

Can be done in one of two ways:

1) Regular file descriptor, just fget()
2) Registered ring, index our own table for that

In preparation for adding another register use of needing to get a ctx
from a file descriptor, abstract out this helper and use it in the main
register syscall as well.

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