net: Allow MSG_EOR in each msghdr of sendmmsg

This patch allows setting MSG_EOR in each individual msghdr passed
in sendmmsg. This allows a sendmmsg to send multiple messages when
using SOCK_SEQPACKET.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>