bcache: use MAX_CACHES_PER_SET instead of magic number 8 in __bch_bucket_alloc_set

Current cache_set has MAX_CACHES_PER_SET caches most, and the macro
is used for
"
	struct cache *cache_by_alloc[MAX_CACHES_PER_SET];
"
in the define of struct cache_set.

Use MAX_CACHES_PER_SET instead of magic number 8 in
__bch_bucket_alloc_set.

Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>