Tasks can fork or die while we try to seize them. It's much more simpler
and much more reliably to unseize what was seized and walk the tree again
in case of some tree check failed.

Yes, this makes it impossible to suspend a big tree which constantly forks :(
but I'd prefer fixing more urgent issues before.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>