btrfs: build tags

correct filename and make the tag more readable

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)

runtime/runtime_btfs.go → runtime/runtime_btrfs.go

@@ -1,4 +1,4 @@
-// +build !no_btrfs
+// +build !exclude_graphdriver_btrfs
 
 package runtime