pgd huge page is not supported yet, let's use the generic definition
in linux/pgtable.h.
And also update the BUILD_BUG_ON() checking for pgd_leaf() in
pti_user_pagetable_walk_p4d() because pgd_leaf() returns boolean value.
Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
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