[TCP]: Avoid two divides in __tcp_grow_window()

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

Using right shifts is OK here and less expensive.

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