bcache: print written and keys in trace_bcache_btree_write

It's useful to dump written block and keys on btree write, this patch
add them into trace_bcache_btree_write.

Signed-off-by: Guoju Fang <fangguoju@gmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>