MAINTAINERS: Add Jan Kara for filesystem DAX

Jan has been developing and reviewing filesystem DAX related changes for
quite a while now, and has agreed to help maintain this code going forward.

Thanks, Jan!

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>