irmap: Don't let proc-parse free handle's mem before caching

Bug was introduced by on-disk-irmap-cache patch. The proc-parse
routine allocates memory for handle, calls ->cb then frees handle.

The problem is that the cb in case of pre-dump saves the handle
for future reference. So, in this future handle's mem happen to
be corrupted.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>