overlayfs: rudimentary test support

overlayfs: rudimentary test support

Adding basic overlayfs support to fstests, it doesn't test anything
overlayfs specific, but runs existing tests on top of overlayfs. It's
following the path from Eric's patchset and Zab's review back in Mar.

A new fstype "overlay" is added, and TEST_DEV/SCRATCH_DEV are required
to be fs paths, and overlayfs is mounted at TEST_DIR/SCRATCH_MNT, so
tests can be run there.

To test overlayfs, setup config as something like the following

TEST_DEV=/mnt/ovl/test
TEST_DIR=/mnt/testarea/test
SCRATCH_DEV=/mnt/ovl/scratch
SCRATCH_MNT=/mnt/testarea/scratch

then run

./check -overlay -g auto

Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>