Make sure we will not overflow shinfo->gso_segs

Minimal TCP MSS size is 8 bytes, and shinfo->gso_segs
is a 16bit field.

TCP_MIN_GSO_SIZE is currently defined in include/net/tcp.h,
it seems cleaner to not bring tcp details into include/linux/netdevice.h
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NAlexander Duyck <alexanderduyck@fb.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>