MIPS: check return value of pgtable_pmd_page_ctor

+. According to Documentation/vm/split_page_table_lock, handle failure
of pgtable_pmd_page_ctor

+. Use GFP_KERNEL_ACCOUNT instead of GFP_KERNEL|__GFP_ACCOUNT

+. Adjust coding style

Fixes: ed914d48

 ("MIPS: add PMD table accounting into MIPS')
Reported-by: Joshua Kinard <kumba@gentoo.org>
Signed-off-by: Huang Pei <huangpei@loongson.cn>
Reviewed-by: Joshua Kinard <kumba@gentoo.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>