mount: allow to dump content even if a part of fs is overmounted (v3)

for that the mount point is bind-mounted in a temporary place.

v2: * check, that the fs you get access to at the end is _really_ the
      one you wanted to
    * use switch_ns/restore_ns helpers
v3: reuse code of __open_mountpoint and a few small cleanups

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>