fs_mgt: Fix remount failure in case of non-overlayfs system

$ adb remount
W DM_DEV_STATUS failed for scratch: No such device or address
E Can not mount overlayfs for partitions: No such file or directory
/system/bin/remount exited with status 8
remount failed

Bug: 138407617
Test: $ adb remount

Change-Id: I0f359071234b57324ad2324905b5239dbfa44d01
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>