overlay: nicer report when features are not supported

overlay: nicer report when features are not supported

Commit ea7ad43 ("fstests: implement require of multiple overlayfs
features") changed the message when tests are not run due to missing
overlayfs feature.

Restore the check for existing module param before trying to mount
which restores the old message format, e.g.:

[not run] feature 'metacopy' not supported by overlay

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