need it to check if we can bindmount from external mount

note: when migrating from criu with patch to criu without,
external mount mapping won't work, we do not support it.

v2: s/real_root/ext_real_root/
v4: add comment
v5: use ext_key field for mapping, put NO_ROOT_MOUNT in root
for old externals for which we do not have it.
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>