MAINTAINERS: update GRE DEMUX patterns

Commit c50cd357 ("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>