hex_dump(): fix address printing

 - Fix printing the same address on every line (ie p+i, not p).
 - Use GNU %p to print pointer, to avoid messing with width etc.
   Side effect: 0x is added, hope that's ok.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>