mm: kasan: do not panic if both panic_on_warn and kasan_multishot set

KASAN errors will currently trigger a panic when panic_on_warn is set.
This renders kasan_multishot useless, as further KASAN errors won't be
reported if the kernel has already paniced.  By making kasan_multishot
disable this behaviour for KASAN errors, we can still have the benefits of
panic_on_warn for non-KASAN warnings, yet be able to use kasan_multishot.

This is particularly important when running KASAN tests, which need to
trigger multiple KASAN errors: previously these would panic the system if
panic_on_warn was set, now they can run (and will panic the system should
non-KASAN warnings show up).

Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Patricia Alfonso <trishalfonso@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20200915035828.570483-6-davidgow@google.com
Link: https://lkml.kernel.org/r/20200910070331.3358048-6-davidgow@google.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>