loop: Only change blocksize when needed.

Return early in loop_set_block_size() if the requested block size is
identical to the one we already have; this avoids expensive calls to
freeze the block queue.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martijn Coenen <maco@android.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>