The patch removes page->virtual for all architectures which do not
define WANT_PAGE_VIRTUAL.  Hash for it instead.

Possibly we could define WANT_PAGE_VIRTUAL for CONFIG_HIGHMEM4G, but it
seems unlikely.

A lot of the pressure went off kmap() and page_address() as a result of
the move to kmap_atomic().  That should be the preferred way to address
CPU load in the set_page_address() and page_address() hashing and
locking.

If kmap_atomic is not usable then the next best approach is for users
to cache the result of kmap() in a local rather than calling
page_address() repeatedly.

One heavy user of kmap() and page_address() is the ext2 directory code.

On a 7G Quad PIII, running four concurrent instances of

	while true
	do
		find /usr/src/linux > /dev/null
	done

on ext2 with everything cached, profiling shows that the new hashed
set_page_address() and page_address() implementations consume 0.4% and
1.3% of CPU time respectively.   I think that's OK.