LoongArch: Refactor bug_handler() implementation

1. Early return for user mode triggered exception with all types.
2. Give a chance to call fixup_exception() for default types (like
   S390).

Signed-off-by: Huacai Chen <[email protected]>

Author: chenhuacai

arch/loongarch/kernel/traps.c

@@ -597,17 +597,24 @@ int is_valid_bugaddr(unsigned long addr)
 
 static void bug_handler(struct pt_regs *regs)
 {
+	if (user_mode(regs)) {
+		force_sig(SIGTRAP);
+		return;
+	}
+
 	switch (report_bug(regs->csr_era, regs)) {
 	case BUG_TRAP_TYPE_BUG:
-	case BUG_TRAP_TYPE_NONE:
-		die_if_kernel("Oops - BUG", regs);
-		force_sig(SIGTRAP);
+		die("Oops - BUG", regs);
 		break;
 
 	case BUG_TRAP_TYPE_WARN:
 		/* Skip the BUG instruction and continue */
 		regs->csr_era += LOONGARCH_INSN_SIZE;
 		break;
+
+	default:
+		if (!fixup_exception(regs))
+			die("Oops - BUG", regs);
 	}
 }