fs_mgr: Fix EnsurePathMounted with a given mount_point.

The expected behavior is to ensure the path mounted at the given mount
point, instead of the one in fstab.

Test: Boot into taimen recovery. Choose "Mount /system" from menu. Check
      that system image is mounted at /mnt/system.
Change-Id: I07128e34bc47561afee8164611709a2b5393f9b7