xfs: react to fsdax failure notifications on the rt device

xfs: react to fsdax failure notifications on the rt device

Now that we have reverse mapping for the realtime device, use the
information to kill processes that have mappings to bad pmem.  This
requires refactoring the existing routines to handle rtgroups or AGs;
and splitting out the translation function to improve cohesion.
Also make a proper header file for the dax holder ops.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>