mkfs.xfs: create filesystems with reverse-mappings

mkfs.xfs: create filesystems with reverse-mappings

Originally-From: Dave Chinner <dchinner@redhat.com>

Create v5 filesystems with rmapbt turned on.  Document the rmapbt
options to mkfs, and initialize the extra field we added for reflink
support.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
[darrick.wong@oracle.com: split patch, add commit message and extra fields]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>