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: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>