xfs: teach xfile to pass back direct-map pages to caller

Certain xfile array operations (such as sorting) can be sped up quite a
bit by allowing xfile users to grab a page to bulk-read the records
contained within it.  Create helper methods to facilitate this.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: Dave Chinner <dchinner@redhat.com>