tcp: Honor the eor bit in tcp_mtu_probe

Avoid SKB coalescing if eor bit is set in one of the relevant
SKBs.

Fixes: c134ecb8 ("tcp: Make use of MSG_EOR in tcp_sendmsg")
Signed-off-by: Ilya Lesokhin <ilyal@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>