RISC-V: KVM: Avoid re-acquiring memslot in kvm_riscv_gstage_map()

RISC-V: KVM: Avoid re-acquiring memslot in kvm_riscv_gstage_map()

The caller has already passed in the memslot, and there are
two instances `{kvm_faultin_pfn/mark_page_dirty}` of retrieving
the memslot again in `kvm_riscv_gstage_map`, we can replace them
with `{__kvm_faultin_pfn/mark_page_dirty_in_slot}`.

Signed-off-by: Quan Zhou <zhouquan@iscas.ac.cn>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/50989f0a02790f9d7dc804c2ade6387c4e7fbdbc.1749634392.git.zhouquan@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>