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>