xfs: give xfs_rmap_intent its own perag reference

xfs: give xfs_rmap_intent its own perag reference

Source kernel commit: c13418e8eb375872ad297aeec5fa26277febc155

Give the xfs_rmap_intent a passive reference to the perag structure
data.  This reference will be used to enable scrub intent draining
functionality in subsequent patches.  The space we're (reverse) mapping
is already allocated, so we need to be able to operate even if the AG is
being shrunk or offlined.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>