io_uring: Support calling io_uring_register with a registered ring fd

Add a new flag IORING_REGISTER_USE_REGISTERED_RING (set via the high bit
of the opcode) to treat the fd as a registered index rather than a file
descriptor.

This makes it possible for a library to open an io_uring, register the
ring fd, close the ring fd, and subsequently use the ring entirely via
registered index.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Link: https://lore.kernel.org/r/f2396369e638284586b069dbddffb8c992afba95.1676419314.git.josh@joshtriplett.org


[axboe: remove extra high bit clear]
Signed-off-by: Jens Axboe <axboe@kernel.dk>