fs_mgr: overlayfs: ReadFstabFromFile touches errno

(cherry picked from commit 98a0128e)

Regression from commit 77c28476
("Remove the mount points defined in skip_mount.cfg from
ReadDefaultFstab()") resulted in ReadFstabFromFile to report
errno ENOENT even upon success, preventing clear error propagation
for real failures (ReadFstabFromFile would _always_ report ENOENT).

The bad error propagation resulted in an adb remount failure.

Added immunization against future adjustments that may also
result in a similar regression in adb remount behaviors.

Test: adb-remount-test.sh
Bug: 132594161
Bug: 128961335
Change-Id: Icf5d48bbfc6d938d4b9657ca8004a6a6ddaab5a6