riscv, perf: Add arch specific perf_arch_bpf_user_pt_regs

RISC-V was missing a proper perf_arch_bpf_user_pt_regs macro for
CONFIG_PERF_EVENT builds.
Signed-off-by: Björn Töpel <bjorn.topel@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191216091343.23260-10-bjorn.topel@gmail.com

arch/riscv/include/asm/perf_event.h

@@ -82,4 +82,8 @@ struct riscv_pmu {
 	int		irq;
 };
 
+#ifdef CONFIG_PERF_EVENTS
+#define perf_arch_bpf_user_pt_regs(regs) (struct user_regs_struct *)regs
+#endif
+
 #endif /* _ASM_RISCV_PERF_EVENT_H */