flowcache: Bring net/core/flow.c under IPsec maintain scope

As flow cache is mainly manipulated from IPsec.
Signed-off-by: NFan Du <fan.du@windriver.com>
Signed-off-by: NSteffen Klassert <steffen.klassert@secunet.com>

MAINTAINERS

@@ -5992,6 +5992,7 @@ L:	netdev@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/klassert/ipsec.git
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/klassert/ipsec-next.git
 S:	Maintained
+F:	net/core/flow.c
 F:	net/xfrm/
 F:	net/key/
 F:	net/ipv4/xfrm*