Appoint myself page cache maintainer

This feels like a sufficiently distinct area of responsibility to be
worth separating out from both MM and VFS.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>