xfs: give xfs_refcount_intent its own perag reference

xfs: give xfs_refcount_intent its own perag reference

Source kernel commit: 00e7b3bac1dc8961bd5aa9d39e79131c6bd81181

Give the xfs_refcount_intent a passive reference to the perag structure
data.  This reference will be used to enable scrub intent draining
functionality in subsequent patches.  Any space being modified by a
refcount intent 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>