More work. Work work work.

read()/write() paths should probably work now. mmap() paths are broken,
and I discovered that my original test case only did write faults on
shared pages *after* a read fault; so the original patches would have
blown up on write faults to not present MAP_SHARED pages.

So, need to write code to do the mappings. Also need to correct
arguments for all callers of find_get_pages{,_contig,_tagged}() and
figure out if we actually care about the last two.

Once mmap() paths work, we should be nearly done here. Just need to add
in code to track numbers related to our stuff in /proc/vmstat, and do a
whole bunch of testing.