sparc64: Like x86 we should check current->mm during perf backtrace generation.

If the MM is not active, only report the top-level PC.  Do not try to
access the address space.
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc/kernel/perf_event.c

@@ -1738,8 +1738,6 @@ static void perf_callchain_user_64(struct perf_callchain_entry *entry,
 {
 	unsigned long ufp;
 
-	perf_callchain_store(entry, regs->tpc);
-
 	ufp = regs->u_regs[UREG_I6] + STACK_BIAS;
 	do {
 		struct sparc_stackf *usf, sf;
@@ -1760,8 +1758,6 @@ static void perf_callchain_user_32(struct perf_callchain_entry *entry,
 {
 	unsigned long ufp;
 
-	perf_callchain_store(entry, regs->tpc);
-
 	ufp = regs->u_regs[UREG_I6] & 0xffffffffUL;
 	do {
 		struct sparc_stackf32 *usf, sf;
@@ -1780,6 +1776,11 @@ static void perf_callchain_user_32(struct perf_callchain_entry *entry,
 void
 perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
 {
+	perf_callchain_store(entry, regs->tpc);
+
+	if (!current->mm)
+		return;
+
 	flushw_user();
 	if (test_thread_flag(TIF_32BIT))
 		perf_callchain_user_32(entry, regs);