[PATCH] fix schedstats null deref in sched_exec

In sched_exec, schedstat_inc will dereference a null pointer if no domain
is found with the SD_BALANCE_EXEC flag set.  This was exposed during
testing of the previous patches where cpus are temporarily attached to a
dummy domain without SD_BALANCE_EXEC set.
Signed-off-by: Nathan Lynch <nathanl@austin.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -1727,8 +1727,8 @@ void sched_exec(void)
 		if (tmp->flags & SD_BALANCE_EXEC)
 			sd = tmp;
 
-	schedstat_inc(sd, sbe_attempts);
 	if (sd) {
+		schedstat_inc(sd, sbe_attempts);
 		new_cpu = find_idlest_cpu(current, this_cpu, sd);
 		if (new_cpu != this_cpu) {
 			schedstat_inc(sd, sbe_pushed);