KVM: RISC-V: Fix wrong usage of PGDIR_SIZE to check page sizes

At the moment, riscv only supports PMD and PUD hugepages. For sv39,
PGDIR_SIZE == PUD_SIZE but not for sv48 and sv57. So fix this by changing
PGDIR_SIZE into PUD_SIZE.

Fixes: 9d05c1fe ("RISC-V: KVM: Implement stage2 page table programming")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>