mount: Don't use /proc/self to access mount points

If a process is in another pidns than /proc, the link /proc/self doesn't
work.

(00.061569) Error (mount.c:558): Can't bind-mount
46:/zdtm/live/static/tempfs.test to /tmp/cr-tmpfs.gBVwTb: No such file
or directory

But since we've switched to the mount namespace (with setns) we
can just go an open the path by its name.

Reported-by: Urgen Sherpa <urgen.sherpa@nepallink.net>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>