x86/boot: Fix zap_low_mappings() to map less of the trampoline

x86/boot: Fix zap_low_mappings() to map less of the trampoline

Regular data access into the trampoline is via the directmap.

As now discussed quite extensively in asm/trampoline.h, the trampoline is
arranged so that only the AP and S3 paths need an identity mapping, and that
they fit within a single page.

Right now, PFN_UP(trampoline_end - trampoline_start) is 2, causing more than
expected of the trampoline to be mapped.  Cut it down just the single page it
ought to be.

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