When running with INVARIANTS, the kernel contains extra checks. However,
these assumptions may not hold true once we've panic'd. Therefore, the
checks hold less value after a panic. Additionally, if one of the checks
fails while we are already panic'd, this creates a double-panic which can
interfere with debugging the original panic.

Therefore, this commit allows an administrator to suppress a response to
KASSERT checks after a panic by setting a tunable/sysctl. The
tunable/sysctl (debug.kassert.suppress_in_panic) defaults to being
enabled.

Reviewed by: kib
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D12920