MAINTAINERS: add kernfs entry

My kernfs patch slipped through because I didn't know which maintainer
to CC. Have been told it's gkh. Add an entry, and sort the file patterns
while we are here.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>