util-linux: Add fix to isolate test fstab entries using CUSTOM_FSTAB

util-linux: Add fix to isolate test fstab entries using CUSTOM_FSTAB

During ptest execution, util-linux adds mount entries in /etc/fstab
and runs `mount -a`, which mounts all available entries from
/etc/fstab. This can cause unintended mounts that are unrelated to
the test, leading to incorrect test behavior.

To avoid this, upstream util-linux introduced a mechanism using
CUSTOM_FSTAB,which isolates test-specific fstab entries. Only entries
listed in CUSTOM_FSTAB are mounted during test execution, ensuring
tests do not interfere with or depend on the system's /etc/fstab.

This commit backports below upstream changes to use CUSTOM_FSTAB.
https://github.com/util-linux/util-linux/commit/ed3d33faff17fb702a3acfca2f9f24e69f4920de
https://github.com/util-linux/util-linux/commit/b1580bd760519a2cf052f023057846e54de47484
https://github.com/util-linux/util-linux/commit/6aa8d17b6b53b86a46c5da68c02a893113130496

Signed-off-by: Virendra Thakur <virendra.thakur@kpit.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>