The test creates a file bindmount in criu mntns and binds it into test
mntns, this external file bindmount is autodetected and restored via
"--external mnt[]" criu option.
Note: In previous patch we fix the problem on this code path where file
bindmount restore fails as there is excess "/" in source path.
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>