We have seen several cases recently where we appear to get a double-fault:
We have an original panic. Then, instead of writing the core to the dump
device, the kernel has a second panic: "smp_targeted_tlb_shootdown:
interrupts disabled". This change is an attempt to fix that second panic.

When the other CPUs are stopped, we can't notify them of the TLB shootdown,
so we skip that operation. However, when the CPUs come back up, we
invalidate the TLB to ensure they correctly observe any changes to the
page mappings.

Reviewed by: kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D9786