security: don't use a negative Opt_err token index

The code uses a bitmap to check for duplicate tokens during parsing, and
that doesn't work at all for the negative Opt_err token case.

There is absolutely no reason to make Opt_err be negative, and in fact
it only confuses things, since some of the affected functions actually
return a positive Opt_xyz enum _or_ a regular negative error code (eg
-EINVAL), and using -1 for Opt_err makes no sense.

There are similar problems in ima_policy.c and key encryption, but they
don't have the immediate bug wrt bitmap handing, and ima_policy.c in
particular needs a different patch to make the enum values match the
token array index.  Mimi is sending that separately.

Reported-by:  <syzbot+a22e0dc07567662c50bc@syzkaller.appspotmail.com>
Reported-by: Eric Biggers <ebiggers@kernel.org>
Fixes: 5208cc83 ("keys, trusted: fix: *do not* allow duplicate key options")
Fixes: 00d60fd3

 ("KEYS: Provide keyctls to drive the new key type ops for asymmetric keys [ver #2]")
Cc: James Morris James Morris <jmorris@namei.org>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>