xfs: fix per-ag reference counting in inode reclaim tree walking

The walk fails to decrement the per-ag reference count when the
non-blocking walk fails to obtain the per-ag reclaim lock, leading
to an assert failure on debug kernels when unmounting a filesystem.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/linux-2.6/xfs_sync.c

@@ -853,6 +853,7 @@ xfs_reclaim_inodes_ag(
 		if (trylock) {
 			if (!mutex_trylock(&pag->pag_ici_reclaim_lock)) {
 				skipped++;
+				xfs_perag_put(pag);
 				continue;
 			}
 			first_index = pag->pag_ici_reclaim_cursor;

fs/xfs/xfs_mount.c

@@ -275,6 +275,7 @@ xfs_free_perag(
 		pag = radix_tree_delete(&mp->m_perag_tree, agno);
 		spin_unlock(&mp->m_perag_lock);
 		ASSERT(pag);
+		ASSERT(atomic_read(&pag->pag_ref) == 0);
 		call_rcu(&pag->rcu_head, __xfs_free_perag);
 	}
 }