io_uring: add helpers for 2 level table alloc

Some parts like fixed file table use 2 level tables, factor out helpers
for allocating/deallocating them as more users are to come.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1709212359cd82eb416d395f86fc78431ccfc0aa.1623634181.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>