xfs: create helpers to scrub a metadata btree

Create helper functions and tracepoints to deal with errors while
scrubbing a metadata btree.

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 924ad45..36396157 100644
--- a/fs/xfs/Makefile
+++ b/fs/xfs/Makefile
@@ -143,6 +143,7 @@
 
 xfs-y				+= $(addprefix scrub/, \
 				   trace.o \
+				   btree.o \
 				   common.o \
 				   scrub.o \
 				   )