ARM: perf: Check that current->mm is alive before getting user callchain

An event may occur when an mm is already released.

As per commit 20afc60f
 'x86, perf: Check that current->mm is alive before getting user callchain'
Acked-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NJean Pihet <jean.pihet@linaro.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm/kernel/perf_event.c

@@ -590,6 +590,10 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
 	}
 
 	perf_callchain_store(entry, regs->ARM_pc);
+
+	if (!current->mm)
+		return;
+
 	tail = (struct frame_tail __user *)regs->ARM_fp - 1;
 
 	while ((entry->nr < PERF_MAX_STACK_DEPTH) &&