mount: Keep external->val on mi

Right now struct mount_info only keeps the ext_mount_map for ->val
field. Since we're planning to switch onto --external engine we
won't have this struct and thus only char *is OK.

travis-ci: success for mnt: Deprecate --ext-mount-map for --external
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>