[MIPS] Make prepare_frametrace() not clobber v0

Since lmo commit 323a380bf9e1a1679a774a2b053e3c1f2aa3f179 ("Simplify
dump_stack()") made prepare_frametrace() always inlined, using $2 (v0)
in __asm__ is not safe anymore.  We can use $1 (at) instead.  Also we
should use "dla" instead of "la" for 64-bit kernel.
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/traps.c

@@ -161,16 +161,20 @@ static void show_stacktrace(struct task_struct *task, struct pt_regs *regs)
 static __always_inline void prepare_frametrace(struct pt_regs *regs)
 {
 	__asm__ __volatile__(
-		"1: la $2, 1b\n\t"
+		".set push\n\t"
+		".set noat\n\t"
 #ifdef CONFIG_64BIT
-		"sd $2, %0\n\t"
+		"1: dla $1, 1b\n\t"
+		"sd $1, %0\n\t"
 		"sd $29, %1\n\t"
 		"sd $31, %2\n\t"
 #else
-		"sw $2, %0\n\t"
+		"1: la $1, 1b\n\t"
+		"sw $1, %0\n\t"
 		"sw $29, %1\n\t"
 		"sw $31, %2\n\t"
 #endif
+		".set pop\n\t"
 		: "=m" (regs->cp0_epc),
 		"=m" (regs->regs[29]), "=m" (regs->regs[31])
 		: : "memory");