parasite: unprotect VMA-s if one of DUMPPAGES operations failed

PARASITE_CMD_DUMPPAGES is called many times and the parasite args
contains an array with vmas at this time, so VMAs can be unprotected in
error case

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>