x86/mm/pageattr: Use __KERNEL_PAGE_EXEC for kernel pgtable

_KERNPG_TABLE isn't the correct pgprot value to use for the PTE, it's
only suitable for the upper levels of the page table structures
(TRUE?).

One benefit is that _PAGE_GLOBAL is now set in the ptes created in
kernel_map_pages_in_pgd() just like if large pages get split later.

Cc: Borislav Petkov <bp@suse.de>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>