MINOR: buffer: introduce b_realign_if_empty()

Many places deal with buffer realignment after data removal. The method
is always the same : if the buffer is empty, set its pointer to the origin.
Let's have a function for this so that we have less code to change with the
new API.

include/common/buf.h

@@ -322,6 +322,13 @@ static inline void b_set_data(struct buffer *b, size_t len)
 	}
 }
 
+/* b_realign_if_empty() : realigns a buffer if it's empty */
+static inline void b_realign_if_empty(struct buffer *b)
+{
+	if (!b_data(b))
+		b->p = b->data;
+}
+
 
 #endif /* _COMMON_BUF_H */