ubi: block: Fix missing blk_mq_end_request

Switching to BLK_MQ_F_BLOCKING wrongly removed the call to
blk_mq_end_request(). Add it back to have our IOs finished

Fixes: 91cc8fbc

 ("ubi: block: set BLK_MQ_F_BLOCKING")
Analyzed-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Daniel Palmer <daniel@0x0f.com>
Link: https://lore.kernel.org/linux-mtd/CAHk-=wi29bbBNh3RqJKu3PxzpjDN5D5K17gEVtXrb7-6bfrnMQ@mail.gmail.com/
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>