sched: micro-optimize mmdrop()

micro-optimize mmdrop(). Improves schedule()'s assembly a bit.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -1514,7 +1514,7 @@ extern struct mm_struct * mm_alloc(void);
 extern void FASTCALL(__mmdrop(struct mm_struct *));
 static inline void mmdrop(struct mm_struct * mm)
 {
-	if (atomic_dec_and_test(&mm->mm_count))
+	if (unlikely(atomic_dec_and_test(&mm->mm_count)))
 		__mmdrop(mm);
 }