lguest: restore boot speed

lguest is dumb and drops *all* the pagetables for set_pte (which is
only used for kernel mapping manipulation, so it's OK without highmem).

But it's used a lot in boot, too.  As a guest optimization, we
suppressed this flushing until the first page switch.  Now we have
initial_page_table, that happens much earlier, so extend the heuristic
to wait until we switch to something other than the swapper_pg_dir or
initial_page_table.

As measured on my laptop under kvm, this dropped the time-to-mount-root
from 48 seconds to 4.3 seconds.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>