generic: handle fs.verity.require_signatures being enabled

generic: handle fs.verity.require_signatures being enabled

Most of the fs-verity tests fail if the fs.verity.require_signatures
sysctl has been set to 1.  Update them to set this sysctl to 0 at the
beginning of the test and restore it to its previous value at the end.

generic/577 intentionally sets this sysctl to 1.  Make it restore the
previous value at the end of the test rather than assuming it was 0.

Also simplify _require_fsverity_builtin_signatures() to just check for
the presence of the file /proc/sys/fs/verity/require_signatures rather
than check whether the fs-verity keyring is listed in /proc/keys.

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