[IPV4] ip_gre: sendto/recvfrom NBMA address

When GRE tunnel is in NBMA mode, this patch allows an application to use
a PF_PACKET socket to:
- send a packet to specific NBMA address with sendto()
- use recvfrom() to receive packet and check which NBMA address it came from

This is required to implement properly NHRP over GRE tunnel.

Signed-off-by: Timo Teras <timo.teras@iki.fi>
Acked-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>