Allow physically non-contiguous chain frames allocation in mps(4)/mpr(4).

Chain frames required to satisfy all 2K of declared I/Os of 128KB each take
more then a megabyte of a physical memory, all of which existing code tries
allocate as physically contiguous. This patch removes that physical
contiguousness requirement, leaving only virtual contiguousness. I was
thinking about other ways of allocation, but the less granular allocation
becomes, the bigger is the overhead and/or complexity, reaching about 100%
overhead if allocate each frame separately.

The patch also bumps the chain frames hard limit from 2K to 16K. It is more
than enough for the case of default REQ_FRAMES and MAXPHYS (the drivers will
allocate less than that automatically), while in case of increased MAXPHYS
it will control maximal memory usage.

Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D14420