RISC-V: KVM: Add support for SBI_FWFT_POINTER_MASKING_PMLEN

RISC-V: KVM: Add support for SBI_FWFT_POINTER_MASKING_PMLEN

Pointer masking is controlled through a WARL field in henvcfg. Expose
the feature only if at least one PMLEN value is supported for VS-mode.
Allow the VMM to block access to the feature by disabling the Smnpm ISA
extension in the guest.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20250111004702.2813013-3-samuel.holland@sifive.com
Signed-off-by: Anup Patel <anup@brainfault.org>