Teach mps(4) and mpr(4) drivers to autotune chain frames.

This is a first part of the change. It makes the drivers to calculate
the required number of chain frames to satisfy worst case scenarios, but
it does not change existing overly strict limits on them. The next step
will be to rewrite the allocator to not require megabytes of physically
contiguous address space, that may be problematic if done after boot,
after doing which the limits can be removed. Until that this code can
just correct user set limits, if they are set too high.

Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D14261