adb-remount-test.sh: Make devt errors warnings instead

OverlayFS on Device Mapper on Dynamic Partition / Split Fiemap doesn't
seem to report stable dev_t values of the underlying "wrapped" device.

For example, when not using overlayFS, the dev_t of a file in /system
may be something like 253:8, a dm-verity wrapped device (253 is device
mapper's major id).
When overlayFS is enabled, the /system mount point is mounted with a
overlayFS device wrapping a dm-linear device wrapping the actual storage
device. In this case, stat-ing a file in /system shows the dev_t to be
something like 0:23. Not only is the major id not correct, the minor id
is not stable across reboots, too.

For now, suppress the error and make it a warning so it don't report
false negatives. We may want to turn this check back on if the overlayfs
driver is fixed of this issue.

Bug: 165925766
Test: adb-remount-test.sh
Change-Id: I035caec3e00d179f227850cbc9835fb3aedf88e5