[TCP]: Avoid a divide in tcp_mtu_probing()

tcp_mtu_to_mss() being signed, compiler might emit an integer divide
to compute tcp_mtu_to_mss()/2 .

Using a right shift is OK here and less expensive.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>