block: missing bio_put following submit_bio_wait

submit_bio_wait() gives the caller an opportunity to examine
struct bio and so expects the caller to issue the put_bio()

This fixes a memory leak reported by a few people in 4.7-rc2
kmemleak report after 9082e87b

 ("block: remove struct bio_batch")

Signed-off-by: Shaun Tancheff <shaun.tancheff@seagate.com>
Tested-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Larry <Finger@lwfinger.net>
Tested-by: David Drysdale <drysdale@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>