arm64/sve: Add stub for sve_max_virtualisable_vl()

Fixes build problems for configurations with KVM enabled but SVE disabled.
Reported-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NMark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211022141635.2360415-2-broonie@kernel.orgSigned-off-by: NWill Deacon <will@kernel.org>

arch/arm64/include/asm/fpsimd.h

@@ -215,6 +215,11 @@ static inline void fpsimd_release_task(struct task_struct *task) { }
 static inline void sve_sync_to_fpsimd(struct task_struct *task) { }
 static inline void sve_sync_from_fpsimd_zeropad(struct task_struct *task) { }
 
+static inline int sve_max_virtualisable_vl(void)
+{
+	return 0;
+}
+
 static inline int sve_set_current_vl(unsigned long arg)
 {
 	return -EINVAL;