RISC-V: KVM: Separate req and fallback_req masks in make_xfence_request

When handling hfence requests in make_xfence_request(), the current code
uses a single 'actual_req' variable and a single vcpu_mask. If any VCPU
fails to enqueue the hfence data (because its queue is full), the request
falls back to 'fallback_req' for all VCPUs, even if other VCPUs still
have available queue space.

This can cause unnecessary fallback for healthy VCPUs, and more seriously,
those healthy VCPUs will not process their already-enqueued hfence
requests because no 'req' is set for them. As a result, their queues will
quickly become full as well, degrading performance for SMP guests.

Fix this by maintaining two separate bitmaps: one for VCPUs that
successfully enqueued the hfence data (req_vcpu_mask) and another for
those that failed (fallback_req_vcpu_mask). Then send the appropriate
requests to each group. This ensures that fallback is only applied to
VCPUs that actually need it, preserving the efficiency of the normal
path for others.

Fixes: 13acfec2dbcc ("RISC-V: KVM: Add remote HFENCE functions based on VCPU requests")
Signed-off-by: Wang Yechao <wang.yechao255@zte.com.cn>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260731093833.1551341-2-wang.yechao255@zte.com.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

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