MAINTAINERS: Update filesystem-dax and NVDIMM entries

Ross has moved on to other areas.

Matthew and Jan are trusted reviewers for DAX.

Dan is now coordinating filesystem-dax pull requests.

Ira and Keith are now involved with the NVDIMM and Device-DAX
sub-systems.

The linux-nvdimm mailing hosts a patchwork instance for both DAX and
NVDIMM patches.

Cc: Matthew Wilcox <willy@infradead.org>
Acked-by: Keith Busch <keith.busch@intel.com>
Acked-by: Ross Zwisler <zwisler@kernel.org>
Acked-by: Ira Weiny <ira.weiny@intel.com>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>