block: Add default switch case to blk_pm_allow_request() to kill warning

With gcc 4.9.0 and 7.3.0:

    block/blk-core.c: In function 'blk_pm_allow_request':
    block/blk-core.c:2747:2: warning: enumeration value 'RPM_ACTIVE' not handled in switch [-Wswitch]
      switch (rq->q->rpm_status) {
      ^

Convert the return statement below the switch() block into a default
case to fix this.

Fixes: e4f36b24 ("block: fix peeking requests during PM")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -2753,9 +2753,9 @@ static bool blk_pm_allow_request(struct request *rq)
 		return rq->rq_flags & RQF_PM;
 	case RPM_SUSPENDED:
 		return false;
-	}
-
+	default:
 		return true;
+	}
 }
 #else
 static bool blk_pm_allow_request(struct request *rq)