[M108 Migration][WRTjs][VD] Optimize the policy of clear dead mount point

[M108 Migration][WRTjs][VD] Optimize the policy of clear dead mount point

1. The init value of "rtc/memory/WebApp/wrtloadercount" is -1;
and not all loop device is used by wrt, so define the timing
of checking dead mount is 6.

2. Not check mount path is exist or not, bcz WRT move this logic
to app process, if we check it, smack issue will happen.

Reference:
https://review.tizen.org/gerrit/288695/

Change-Id: Ia87b67a59553cc8a8cebe143f3b02849171d4a79
Signed-off-by: zhaosy <shiyusy.zhao@samsung.com>