lguest: use a special 1:1 linear pagetable mode until first switch.

The Host used to create some page tables for the Guest to use at the
top of Guest memory; it would then tell the Guest where this was.  In
particular, it created linear mappings for 0 and 0xC0000000 addresses
because lguest used to switch to its real page tables quite late in
boot.

However, since d50d8fe19 Linux initialized boot page tables in
head_32.S even before the "are we lguest?" boot jump.  So, now we can
simplify things: the Host pagetable code assumes 1:1 linear mapping
until it first calls the LHCALL_NEW_PGTABLE hypercall, which we now do
before we reach C code.

This also means that the Host doesn't need to know anything about the
Guest's PAGE_OFFSET.  (Non-Linux guests might not even have such a
thing).

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

include/linux/lguest.h

@@ -59,8 +59,6 @@ struct lguest_data {
 	unsigned long reserve_mem;
 	/* KHz for the TSC clock. */
 	u32 tsc_khz;
-	/* Page where the top-level pagetable is */
-	unsigned long pgdir;
 
 /* Fields initialized by the Guest at boot: */
 	/* Instruction range to suppress interrupts even if enabled */