[GFS2] Use slab operations for all gfs2_bufdata allocations

The old revoke structure was allocated using kalloc/kfree but
there is a slab cache for gfs2_bufdata, so we should use that
now that the structures have been converted.

This is part two of the patch series to merge the revoke
and gfs2_bufdata structures.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
diff --git a/fs/gfs2/lops.c b/fs/gfs2/lops.c
index cf6fe363..4cbef4c 100644
--- a/fs/gfs2/lops.c
+++ b/fs/gfs2/lops.c
@@ -393,7 +393,7 @@
 		}
 
 		*(__be64 *)(bh->b_data + offset) = cpu_to_be64(bd->bd_blkno);
-		kfree(bd);
+		kmem_cache_free(gfs2_bufdata_cachep, bd);
 
 		offset += sizeof(u64);
 	}