MIPS: add PMD table accounting into MIPS'pmd_alloc_one

This fixes Page Table accounting bug.

MIPS is the ONLY arch just defining __HAVE_ARCH_PMD_ALLOC_ONE alone.
Since commit b2b29d6d

 (mm: account PMD tables like PTE tables),
"pmd_free" in asm-generic with PMD table accounting and "pmd_alloc_one"
in MIPS without PMD table accounting causes PageTable accounting number
negative, which read by global_zone_page_state(), always returns 0.

Signed-off-by: Huang Pei <huangpei@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>