[NET]: kernel-doc fix for sock.h

Fix kernel-doc warning in include/net/sock.h:
Warning(/var/linsrc/linux-2619-rc1-pv//include/net/sock.h:894): No description found for parameter 'rcu'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>