LoongArch: KVM: Use kvm_set_pte() in kvm_flush_pte()

LoongArch: KVM: Use kvm_set_pte() in kvm_flush_pte()

kvm_flush_pte() is the only caller that directly assigns *pte instead
of using the kvm_set_pte() wrapper. Use the wrapper for consistency with
the rest of the file.

No functional change intended.

Cc: stable@vger.kernel.org
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>