Quick fix for initialization of mnt_iosize_max. (This limit controls
mainly clustering and read-ahead.) Copy the initialization from ffs,
and also copy a couple of lines of ffs's nearby style for initialization
order and whitespace.

A correct fix would de-duplicate the initialization and fix bitrot in it
instead of adding another instance of the duplication. Complications to
use the size preferred by the device have been reduced to hard-coding
slightly pessimal and/or inconsistent defaults, using large code that was
almost needed to support the complications.

For msdosfs, the result was that mnt_iosize_max was DFTLPHYS (64K) but is
now MAXPHYS (128K).