xfs: give xfs_extfree_intent its own perag reference

xfs: give xfs_extfree_intent its own perag reference

Source kernel commit: f6b384631e1e3482c24e35b53adbd3da50e47e8f

Give the xfs_extfree_intent an passive reference to the perag structure
data.  This reference will be used to enable scrub intent draining
functionality in subsequent patches.  The space being freed must already
be allocated, so we need to able to run even if the AG is being offlined
or shrunk.

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