MAINTAINERS: update GRE DEMUX patterns

Commit c50cd35 ("net: gre: move GSO functions to gre_offload")
renamed and separated the file into multiple files.  Update the
patterns.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Dmitry Kozlov <xeb@mail.ru>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

MAINTAINERS

@@ -3683,7 +3683,8 @@ GRE DEMULTIPLEXER DRIVER
 M:	Dmitry Kozlov <xeb@mail.ru>
 L:	netdev@vger.kernel.org
 S:	Maintained
-F:	net/ipv4/gre.c
+F:	net/ipv4/gre_demux.c
+F:	net/ipv4/gre_offload.c
 F:	include/net/gre.h
 
 GRETH 10/100/1G Ethernet MAC device driver