Revert "io_uring/rsrc: disallow multi-source reg buffers"

This reverts commit edd47826.

There's really no specific need to disallow multiple sources of buffers,
and io_uring really should not be mandating this by itself. We should
be able to solely rely on GUP making these decisions.

As this also stands in the way of a cleanup where io_uring is the odd
one out, kill it.

Link: https://lore.kernel.org/all/61ded378-51a8-1dcb-b631-fda1903248a9@gmail.com/


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