Change the comment before this block of code, and separate the latter
from the preceding one by an empty line.
Move the loop on phys_avail[] to compute the minimum and maximum memory
physical addresses closer to the initialization of 'low_avail' and
'high_avail', so that it's immediately clear why the loop starts at
2 (and remove the related comment).
While here, fuse the additional loop in the VM_PHYSSEG_DENSE case that
is used to compute the exact physical memory size.
This change suppresses one occurence of detecting whether at least one
of VM_PHYSSEG_DENSE or VM_PHYSSEG_SPARSE is defined at compile time, but
there is still another one in PHYS_TO_VM_PAGE().