io_uring: opcode independent fixed buf import

Fixed buffers are generic infrastructure, make io_import_fixed() opcode
agnostic.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b1e765c8a1c2c913a05a28d2399fc53e1d3cf37a.1655684496.git.asml.silence@gmail.com


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