Fix dumping of /proc folder

CRIU fails to dump processes that have a file descriptor pointing
to the /proc folder, because check_path_remap returns error code
in that case. Fix: return 0 instead.

Signed-off-by: Andrei Tuicu <andrei.tuicu@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>