x86: avoid duplicate running of pud_offset and pmd_offset in one_md_table_init()

Impact: simplify implementation, cleanup

If !(pgd_val(*pgd) & _PAGE_PRESENT) in PAE mode, we need not get value of
pmd_table again.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>