overlay: mount/unmount base fs before/after running tests

overlay: mount/unmount base fs before/after running tests

When TEST/SCRATCH_DEV are configured to the base fs block device,
use this information to mount base fs before running tests,
unmount it after running tests and cycle on _test_cycle_mount
along with the overlay mounts.

This helps catching overlayfs bugs related to leaking objects in
underlying (base) fs.

To preserve expected tests behavior, the semantics are:
- _scratch_mkfs mounts the base fs, cleans all files, creates
  lower/upper dirs and keeps base fs mounted
- _scratch_mount mounts base fs (if needed) and mounts overlay
- _scratch_unmount unmounts overlay and base fs

Tests that use _scratch_unmount to unmount a custom overlay mount
and expect to have access to overlay base dir, were fixed to use
explicit umount $SCRATCH_MNT instead.

The overlay test itself, does not support formatting the base fs,
so config options like MKFS_OPTIONS and FSCK_OPTIONS are ignored
on 'check -overlay'.
The config option MOUNT_OPTIONS is used to mount the base scratch fs.
The config option TEST_FS_MOUNT_OPTS is used to mount the base test fs.
The config option OVERLAY_MOUNT_OPTIONS is used to mount both test and
scratch overlay mounts.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>