pagemap-cache: Use greedy mode if pagemap inaccessible

If criu is running in unprivileged mode and we can't
access dumpee's pagemap file -- simply switch to
greedy mode where all pages are gonna be dumped
regardless of their presence in memory.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Looks-good-to-me: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>