Fix broken audit tests for exec arg len

The "fix" in commit 0b08c5e5 ("audit: Fix check of return value of
strnlen_user()") didn't fix anything, it broke things.  As reported by
Steven Rostedt:

 "Yes, strnlen_user() returns 0 on fault, but if you look at what len is
  set to, than you would notice that on fault len would be -1"

because we just subtracted one from the return value.  So testing
against 0 doesn't test for a fault condition, it tests against a
perfectly valid empty string.

Also fix up the usual braindamage wrt using WARN_ON() inside a
conditional - make it part of the conditional and remove the explicit
unlikely() (which is already part of the WARN_ON*() logic, exactly so
that you don't have to write unreadable code.

Reported-and-tested-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Paul Moore <pmoore@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>