net: fixes for skb_send_sock

A couple fixes to new skb_send_sock infrastructure. However, no users
currently exist for this code (adding user in next handful of patches)
so it should not be possible to trigger a panic with existing in-kernel
code.

Fixes: 306b13eb ("proto_ops: Add locked held versions of sendmsg and sendpage")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>