Adjust KERNEL_ASSERT_UNLOCKED() to not assert during a panic.

Adjust KERNEL_ASSERT_UNLOCKED() to not assert during a panic.

KERNEL_ASSERT_UNLOCKED calls _kernel_lock_held() which returns true
if panicstr || db_active which triggers this assert. Workaround this by
checking them before.

This will alter the following Syzkaller reports:
Reported-by: syzbot+169110a0815838ab5940@syzkaller.appspotmail.com
Reported-by: syzbot+3c2eced405b9de6f79c2@syzkaller.appspotmail.com
OK mpi@