x86/p2m: avoid unnecessary calls of write_p2m_entry_pre() hook

x86/p2m: avoid unnecessary calls of write_p2m_entry_pre() hook

When shattering a large page, we first construct the new page table page
and only then hook it up. The "pre" hook in this case does nothing, for
the page starting out all blank. Avoid 512 calls into shadow code in
this case by passing in INVALID_GFN, indicating the page being updated
is (not yet) associated with any GFN. (The alternative to this change
would be to actually pass in a correct GFN, which can't be all the same
on every loop iteration.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>