Save the last callout function executed on each CPU

Save the last callout function pointer (and its argument) executed
on each CPU for inspection by a debugger. Add a ddb show callout_last
command to show these pointers. Add a kernel module that I used
for testing that command.

Relocate ce_migration_cpu to reduce padding and therefore preserve
the size of struct callout_cpu (320 bytes on amd64) despite the
added members.

This should help diagnose reference-after-free bugs where the
callout's mutex has already been freed when softclock_call_cc
tries to unlock it.

You might hope that the pointer would still be available, but it
isn't. The argument to that function is on the stack (because
softclock_call_cc uses it later), and that might be enough in
some cases, but even then, it's very laborious. A pointer to the
callout is saved right before these newly added fields, but that
callout might have been freed. We still have the pointer to its
associated mutex, and the name within might be enough, but it might
also have been freed.

Reviewed by: markj jhb
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D20794