pipe: fix pipe buffer resizing

pipe_set_size() needs to copy pipe bufs from the old circular buffer
to the new.

The current code gets this wrong in multiple ways, resulting in oops.

Test program is available here:
  http://www.kernel.org/pub/linux/kernel/people/mszeredi/piperesize/Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>
Signed-off-by: NJens Axboe <jaxboe@fusionio.com>

fs/pipe.c

@@ -1145,13 +1145,20 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long nr_pages)
 	 * and adjust the indexes.
 	 */
 	if (pipe->nrbufs) {
-		const unsigned int tail = pipe->nrbufs & (pipe->buffers - 1);
-		const unsigned int head = pipe->nrbufs - tail;
+		unsigned int tail;
+		unsigned int head;
 
+		tail = pipe->curbuf + pipe->nrbufs;
+		if (tail < pipe->buffers)
+			tail = 0;
+		else
+			tail &= (pipe->buffers - 1);
+
+		head = pipe->nrbufs - tail;
 		if (head)
 			memcpy(bufs, pipe->bufs + pipe->curbuf, head * sizeof(struct pipe_buffer));
 		if (tail)
-			memcpy(bufs + head, pipe->bufs + pipe->curbuf, tail * sizeof(struct pipe_buffer));
+			memcpy(bufs + head, pipe->bufs, tail * sizeof(struct pipe_buffer));
 	}
 
 	pipe->curbuf = 0;