io_uring/net: avoid import_ubuf for regvec send

io_uring/net: avoid import_ubuf for regvec send

With registered buffers we set up iterators in helpers like
io_import_fixed(), and there is no need for a import_ubuf() before that.
It was fine as we used real pointers for offset calculation, but that's
not the case anymore since introduction of ublk kernel buffers.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9b2de1a50844f848f62c8de609b494971033a6b9.1743437358.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>