Revert "remount: Remove the "backing" parameter to fs_mgr_overlayfs_setup."

This reverts commit b20aa434a38e7604c42536a16ac637bfc45a2d15.

Bug: 146403375
Reason for revert: broken build 6078429 on git_master on walleye-user

Change-Id: Idc3b1c0ad5228d63298ce84811624b23d0cea434
4 files changed