perf: Fix software migrate events

Stephane asked about PERF_COUNT_SW_CPU_MIGRATIONS and I realized it
was borken:

 > The problem is that the task isn't actually scheduled while its being
 > migrated (obviously), and if its not scheduled, the counters aren't
 > scheduled either, so there's no observing of the fact.
 >
 > A further problem with migrations is that many migrations happen from
 > softirq context, which is nested inside the 'random' task context of
 > whoemever happens to run at that time, similarly for the wakeup
 > migrations triggered from (soft)irq context. All those end up being
 > accounted in the task that's currently running, eg. your 'ls'.

The below cures this by marking a task as migrated and accounting it
on the subsequent sched_in().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched.h

@@ -1356,9 +1356,6 @@ struct task_struct {
 #endif
 
 	struct mm_struct *mm, *active_mm;
-#ifdef CONFIG_COMPAT_BRK
-	unsigned brk_randomized:1;
-#endif
 	/* per-thread vma caching */
 	u32 vmacache_seqnum;
 	struct vm_area_struct *vmacache[VMACACHE_SIZE];
@@ -1381,10 +1378,14 @@ struct task_struct {
 	/* Revert to default priority/policy when forking */
 	unsigned sched_reset_on_fork:1;
 	unsigned sched_contributes_to_load:1;
+	unsigned sched_migrated:1;
 
 #ifdef CONFIG_MEMCG_KMEM
 	unsigned memcg_kmem_skip_account:1;
 #endif
+#ifdef CONFIG_COMPAT_BRK
+	unsigned brk_randomized:1;
+#endif
 
 	unsigned long atomic_flags; /* Flags needing atomic access. */