overlay: prepare for new lowerdir+,datadir+ tests

In preparation to forking tests for new lowerdir+,datadir+ mount options,
prepare a helper to test kernel support and pass datadirs into mount
helpers in overlay/079 test.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>