net: fix uninitialised msghdr->sg_from_iter

Because of how struct msghdr is usually initialised some fields and
sg_from_iter in particular might be left out not initialised, so we
can't safely use it in __zerocopy_sg_from_iter().

For now use the callback only when there is ->msg_ubuf set relying on
the fact that they're used together and we properly zero ->msg_ubuf.

Fixes: ebe73a28 ("net: Allow custom iter handler in msghdr")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Message-Id: <ce8b68b41351488f79fd998b032b3c56e9b1cc6c.1658401817.git.asml.silence@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>