If container has external bindmount given to criu through
--ext-mount-map option by admin, container user can bindmount
subdirs of these external bindmount to somewhere else inside
container creating secondary external bindmounts. Criu we will
fail to restore them as having unreachable sharing. But we can
restore secondary external bindmounts bindmounting them from
primary external bindmount.

v2: s/external_bind/mnt_is_external/, make mnt_is_external bool,
do mnt_is_external without recursion
v3: add debug message on propagate_mount when bind is set
v5: remove hunk of v4 which goes to previous patch, so same as v3
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>