io_uring/net: derive iovec storage later

io_uring/net: derive iovec storage later

Don't read free_iov until right before we need it to import the iovec.
The only place that uses it before that is provided buffer selection,
but it only serves as temporary storage and iovec content is not reused
afterwards, so use a local variable for that.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8bfa7d74c33e37860a724f4e0e96660c25cd4c02.1740569495.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>