MINOR: buffer: implement b_alloc_fast()

This function allocates a buffer and replaces *buf with this buffer. If
no memory is available, &buf_wanted is used instead. No control is made
to check if *buf already pointed to another buffer. The allocated buffer
is returned, or NULL in case no memory is available. The difference with
b_alloc() is that this function only picks from the pool and never calls
malloc(), so it can fail even if some memory is available. It is the
caller's job to refill the buffer pool if needed.

include/common/buffer.h

@@ -422,6 +422,27 @@ static inline struct buffer *b_alloc(struct buffer **buf)
 	return b;
 }
 
+/* Allocates a buffer and replaces *buf with this buffer. If no memory is
+ * available, &buf_wanted is used instead. No control is made to check if *buf
+ * already pointed to another buffer. The allocated buffer is returned, or
+ * NULL in case no memory is available. The difference with b_alloc() is that
+ * this function only picks from the pool and never calls malloc(), so it can
+ * fail even if some memory is available.
+ */
+static inline struct buffer *b_alloc_fast(struct buffer **buf)
+{
+	struct buffer *b;
+
+	*buf = &buf_wanted;
+	b = pool_get_first(pool2_buffer);
+	if (likely(b)) {
+		b->size = pool2_buffer->size - sizeof(struct buffer);
+		b_reset(b);
+		*buf = b;
+	}
+	return b;
+}
+
 /* Releases buffer *buf (no check of emptiness) */
 static inline void __b_drop(struct buffer **buf)
 {