[SOCK] Avoid integer divides where not necessary in include/net/sock.h

Because sk_wmem_queued, sk_sndbuf are signed, a divide per two
may force compiler to use an integer divide.

We can instead use a right shift.

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