io_uring/net: unify how recvmsg and sendmsg copy in the msghdr

[ Upstream commit 52307ac4 ]

For recvmsg, we roll our own since we support buffer selections. This
isn't the case for sendmsg right now, but in preparation for doing so,
make the recvmsg copy helpers generic so we can call them from the
sendmsg side as well.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: 8ede3db5 ("io_uring/net: fix overflow check in io_recvmsg_mshot_prep()")
Signed-off-by: Sasha Levin <sashal@kernel.org>