After commit 1355c31e ("asm-generic: pgalloc: provide generic
pmd_alloc_one() and pmd_free_one()"), the main part to support
PMD split page table lock is in asm-generic/pgalloc.h.

The only change is add pgtable_pmd_page_ctor() into alloc_pmd_late(),
then we could enable ARCH_ENABLE_SPLIT_PMD_PTLOCK for RV64.
Reviewed-by: NAlexandre Ghiti <alex@ghiti.fr>
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NPalmer Dabbelt <palmer@rivosinc.com>