net: fix kernel-doc for sk_filter_rcu_release

Fix kernel-doc warning for sk_filter_rcu_release():

Warning(net/core/filter.c:586): missing initial short description on line:
 * 	sk_filter_rcu_release: Release a socket filter by rcu_head

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc:	"David S. Miller" <davem@davemloft.net>
Cc:	netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>