fs_mgr: errno handling in fs_mgr_overlayfs_setup()

Do not report errno if ENOENT or ENXIO as it is expected when verity
is in fact disabled or not setup on platform.

Reading the default fstab can also result in ENOENT if fstab or dt
are missing on purpose, but if we get an fstab then restore the
errno as we move on.

fs_mgr_has_shared_blocks sets errno, when all we care about is yes
or no answer (EPERM notwithstanding, which indicates do not know).

If no candidates are found to override, and not caused by EPERM,
then suppress errno.

Test: adb-remount-test.sh
Bug: 109821005
Change-Id: If7ae257cb6b738a64ba43d32805760cc292b2fae