blk-mq: silence false positive warnings in hctx_unlock()

In some stupider versions of gcc, it complains:

block/blk-mq.c: In function ‘blk_mq_complete_request’:
./include/linux/srcu.h:175:2: warning: ‘srcu_idx’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  __srcu_read_unlock(sp, idx);
  ^
block/blk-mq.c:620:6: note: ‘srcu_idx’ was declared here
  int srcu_idx;
      ^

which is completely bogus, since we only use srcu_idx when
hctx->flags & BLK_MQ_F_BLOCKING is set, and that's the case where
hctx_lock() has initialized it.

Just set it to '0' in the normal path in hctx_lock() to silence
this annoying warning.

Fixes: 04ced159 ("blk-mq: move hctx lock/unlock into a helper")
Fixes: 5197c05e ("blk-mq: protect completion path with RCU")
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -569,9 +569,11 @@ static void hctx_unlock(struct blk_mq_hw_ctx *hctx, int srcu_idx)
 
 static void hctx_lock(struct blk_mq_hw_ctx *hctx, int *srcu_idx)
 {
-	if (!(hctx->flags & BLK_MQ_F_BLOCKING))
+	if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
+		/* shut up gcc false positive */
+		*srcu_idx = 0;
 		rcu_read_lock();
-	else
+	} else
 		*srcu_idx = srcu_read_lock(hctx->srcu);
 }