riscv: tracing: Fix __write_overflow_field in ftrace_partial_regs()

The size of &regs->a0 is unknown, causing the error:

../include/linux/fortify-string.h:571:25: warning: call to
'__write_overflow_field' declared with attribute warning: detected write
beyond size of field (1st parameter); maybe use struct_group()?
[-Wattribute-warning]

Fix this by wrapping the required registers in pt_regs with
struct_group() and reference the group when doing the offending
memcpy().

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250224-fix_ftrace_partial_regs-v1-1-54b906417e86@rivosinc.com
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>

arch/riscv/include/asm/ftrace.h

@@ -207,7 +207,7 @@ ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs *regs)
 {
 	struct __arch_ftrace_regs *afregs = arch_ftrace_regs(fregs);
 
-	memcpy(&regs->a0, afregs->args, sizeof(afregs->args));
+	memcpy(&regs->a_regs, afregs->args, sizeof(afregs->args));
 	regs->epc = afregs->epc;
 	regs->ra = afregs->ra;
 	regs->sp = afregs->sp;

arch/riscv/include/asm/ptrace.h

@@ -23,14 +23,16 @@ struct pt_regs {
 	unsigned long t2;
 	unsigned long s0;
 	unsigned long s1;
-	unsigned long a0;
-	unsigned long a1;
-	unsigned long a2;
-	unsigned long a3;
-	unsigned long a4;
-	unsigned long a5;
-	unsigned long a6;
-	unsigned long a7;
+	struct_group(a_regs,
+		unsigned long a0;
+		unsigned long a1;
+		unsigned long a2;
+		unsigned long a3;
+		unsigned long a4;
+		unsigned long a5;
+		unsigned long a6;
+		unsigned long a7;
+	);
 	unsigned long s2;
 	unsigned long s3;
 	unsigned long s4;