generic: add an fs-verity stress test

generic: add an fs-verity stress test

Add a stress test for fs-verity.  This tests enabling fs-verity on
multiple files concurrently with concurrent readers on those files
(with reads occurring before, during, and after the fs-verity
enablement), while fsstress is also running on the same filesystem.

I haven't seen any failures from running this on ext4 and f2fs.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>