Before dump:

438 437 0:51 /mtest /zdtm/static/mnt_ext_manual.test rw,relatime - tmpfs zdtm_auto_ext_mnt rw
439 437 0:51 /mtest /zdtm/static/mnt_ext_manual_private_shared_bind.test rw,relatime shared:144 - tmpfs zdtm_auto_ext_mnt rw
440 437 0:51 /mtest /zdtm/static/mnt_ext_manual_bind.test rw,relatime shared:144 - tmpfs zdtm_auto_ext_mnt rw
441 437 0:51 /mtest /zdtm/static/mnt_ext_manual_slave_shared_bind.test rw,relatime shared:145 master:144 - tmpfs zdtm_auto_ext_mnt rw
442 437 0:51 /mtest /zdtm/static/mnt_ext_manual_slave_bind.test rw,relatime master:145 - tmpfs zdtm_auto_ext_mnt rw

>From log: mount mnt_ext_manual_slave_bind is restored as slave of
mnt_ext_manual_slave_shared_bind, which is restored as slave of
mnt_ext_manual_private_shared_bind, which is restored as shared of
mnt_ext_manual_bind, which is restored as private bind of external,
that is exactly the same way mnt_is_external() looks for external.

v2: fix test to cover all cases in mnt_is_external()
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>