autofs: use temporary value for virtual pgrp on dump

Otherwise information about real pid is lost in case of pid_to_virt returned
zero. This is required to output the real pid in error message instead of
always outputting zero (result of pid_to_virt).
IOW, would be nice to know the process pid we failed to find in the tree.

Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>