x86/boot: Don't map 0 during boot

x86/boot: Don't map 0 during boot

In particular, it causes accidental NULL pointer dereferences to go unnoticed.

The majority of the early operation takes place either in Real mode, or
Protected Unpaged mode.  The only bit which requires pagetable mappings is the
trampoline transition into Long mode and jump to the higher mappings, so there
is no need for the whole bottom 2M to be mapped.

Introduce a new l1_bootmap in .init.data, and use it instead of l1_identmap.
The EFI boot path doesn't pass through the trampoline, so doesn't need any
adjustment.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>