xfs: scrub refcount btrees

Plumb in the pieces necessary to check the refcount btree.  If rmap is
available, check the reference count by performing an interval query
against the rmapbt.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
diff --git a/fs/xfs/Makefile b/fs/xfs/Makefile
index 61fb281..77d5dcb 100644
--- a/fs/xfs/Makefile
+++ b/fs/xfs/Makefile
@@ -148,6 +148,7 @@
 				   btree.o \
 				   common.o \
 				   ialloc.o \
+				   refcount.o \
 				   rmap.o \
 				   scrub.o \
 				   )