net/core: add lock context change annotations in net/core/sock.c

__lock_sock() and __release_sock() releases and regrabs lock but
were missing proper annotations. Add it. This removes following
warning from sparse. (Currently __lock_sock() does not emit any
warning about it but I think it is better to add also.)

 net/core/sock.c:1580:17: warning: context imbalance in '__release_sock' - unexpected unlock

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>