dump: move the may_dump() check in seize_task()

It's a bad idea to a group of processes and only then check
rights for this operation. We need to check permissions a soon as
posible to reduce impacts in case of wrong permissions.

In addtion criu doesn't to parse /proc/pid/state and gets all required
infromation from /proc/pid/status.

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