xfs: use xas_for_each_marked in xfs_reclaim_inodes_count

xfs_reclaim_inodes_count iterates over all AGs to sum up the reclaimable
inodes counts.  There is no point in grabbing a reference to the them or
unlock the RCU critical section for each iteration, so switch to the
more efficient xas_for_each_marked iterator.

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