cephfs: add a getErrorIfNegative helper func

Works like the ones already in the other packages.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

cephfs/errors.go

@@ -35,6 +35,16 @@ func getError(e C.int) error {
 	return cephFSError(e)
 }
 
+// getErrorIfNegative converts a ceph return code to error if negative.
+// This is useful for functions that return a usable positive value on
+// success but a negative error number on error.
+func getErrorIfNegative(ret C.int) error {
+	if ret >= 0 {
+		return nil
+	}
+	return getError(ret)
+}
+
 // Public go errors:
 
 var (