io_uring/rsrc: refactor io_import_fixed

io_import_fixed is a mess. Even though we know the final len of the
iterator, we still assign offset + len and do some magic after to
correct for that.

Do offset calculation first and finalise it with iov_iter_bvec at the
end.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2d5107fed24f8b23245ef2ede9a5a7f7c426df61.1744882081.git.asml.silence@gmail.com


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