Pat the watchdog less while producing a coredump. Prior to this change,
we patted the watchdog approximately once per 4KB page of memory. After
this change, we pat the watchdog approximately once per 128MB of memory.
On a sample machine, this translated to patting the watchdog approximately
every 5.4 seconds, which "seems reasonable". We can choose a different
value in the future, if warranted.

This has extensive field experience. It is a performance improvement, and
has not caused any known problems.

Reviewed by: imp, kib
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D14988