riscv, bpf: Fix RV32 broken build, and silence RV64 warning

Commit 252c765b ("riscv, bpf: Add BPF exception tables") only addressed
RV64, and broke the RV32 build [1]. Fix by gating the exception tables code
with CONFIG_ARCH_RV64I.

Further, silence a "-Wmissing-prototypes" warning [2] in the RV64 BPF JIT.

  [1] https://lore.kernel.org/llvm/202111020610.9oy9Rr0G-lkp@intel.com/
  [2] https://lore.kernel.org/llvm/202110290334.2zdMyRq4-lkp@intel.com/

Fixes: 252c765b ("riscv, bpf: Add BPF exception tables")
Signed-off-by: Björn Töpel <bjorn@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Tong Tiangen <tongtiangen@huawei.com>
Link: https://lore.kernel.org/bpf/20211103115453.397209-1-bjorn@kernel.org

arch/riscv/mm/extable.c

@@ -11,7 +11,7 @@
 #include <linux/module.h>
 #include <linux/uaccess.h>
 
-#ifdef CONFIG_BPF_JIT
+#if defined(CONFIG_BPF_JIT) && defined(CONFIG_ARCH_RV64I)
 int rv_bpf_fixup_exception(const struct exception_table_entry *ex, struct pt_regs *regs);
 #endif
 
@@ -23,7 +23,7 @@ int fixup_exception(struct pt_regs *regs)
 	if (!fixup)
 		return 0;
 
-#ifdef CONFIG_BPF_JIT
+#if defined(CONFIG_BPF_JIT) && defined(CONFIG_ARCH_RV64I)
 	if (regs->epc >= BPF_JIT_REGION_START && regs->epc < BPF_JIT_REGION_END)
 		return rv_bpf_fixup_exception(fixup, regs);
 #endif

arch/riscv/net/bpf_jit_comp64.c

@@ -459,6 +459,8 @@ static int emit_call(bool fixed, u64 addr, struct rv_jit_context *ctx)
 #define BPF_FIXUP_OFFSET_MASK   GENMASK(26, 0)
 #define BPF_FIXUP_REG_MASK      GENMASK(31, 27)
 
+int rv_bpf_fixup_exception(const struct exception_table_entry *ex,
+				struct pt_regs *regs);
 int rv_bpf_fixup_exception(const struct exception_table_entry *ex,
 				struct pt_regs *regs)
 {