Replace MAXPHYS by runtime variable maxphys. It is initialized from MAXPHYS by default, but can be also adjusted with the tunable kern.maxphys.
Make b_pages[] array in struct buf flexible. Size b_pages[] for buffer cache buffers exactly to atop(maxbcachebuf) (currently it is sized to atop(MAXPHYS)), and b_pages[] for pbufs sized to atop(maxphys) + 1. +1 for pbufs allow several pbuf consumers, among them vmapbuf(), to use unaligned buffers still sized to maxphys, esp. when such buffers come from userspace (*). Overall, we save significant amount of otherwise wasted memory in b_pages[] for buffer cache buffers, while bumping MAXPHYS to desired high value.
Eliminate all direct uses of the MAXPHYS constant in kernel and driver sources, except a place which initialize maxphys. Some random (and arguably weird) uses of MAXPHYS, e.g. in linuxolator, are converted straight.
Changes to cam/, dev/ahci, dev/ata, dev/mpr, dev/mpt, dev/mvs, dev/siis, where either submitted by, or based on changes by mav.
Suggested by: mav (*)