RISC-V: KVM: Fix inclusion of Smnpm in the guest ISA bitmap

The Smnpm extension requires special handling because the guest ISA
extension maps to a different extension (Ssnpm) on the host side.
commit 1851e7836212 ("RISC-V: KVM: Allow Smnpm and Ssnpm extensions for
guests") missed that the vcpu->arch.isa bit is based only on the host
extension, so currently both KVM_RISCV_ISA_EXT_{SMNPM,SSNPM} map to
vcpu->arch.isa[RISCV_ISA_EXT_SSNPM]. This does not cause any problems
for the guest, because both extensions are force-enabled anyway when the
host supports Ssnpm, but prevents checking for (guest) Smnpm in the SBI
FWFT logic.

Redefine kvm_isa_ext_arr to look up the guest extension, since only the
guest -> host mapping is unambiguous. Factor out the logic for checking
for host support of an extension, so this special case only needs to be
handled in one place, and be explicit about which variables hold a host
vs a guest ISA extension.

Fixes: 1851e7836212 ("RISC-V: KVM: Allow Smnpm and Ssnpm extensions for guests")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20250111004702.2813013-2-samuel.holland@sifive.com
Signed-off-by: Anup Patel <anup@brainfault.org>

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