Drop the globally visible PFERR_NESTED_GUEST_PAGE and replace it with a
more appropriately named is_write_to_guest_page_table().  The macro name
is misleading, because while all nNPT walks match PAGE|WRITE|PRESENT, the
reverse is not true.

No functional change intended.

Link: https://lore.kernel.org/r/20240831001538.336683-3-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>