bcache: replace hard coded number with BUCKET_GC_GEN_MAX

In extents.c:bch_extent_bad(), number 96 is used as parameter to call
btree_bug_on(). The purpose is to check whether stale gen value exceeds
BUCKET_GC_GEN_MAX, so it is better to use macro BUCKET_GC_GEN_MAX to
make the code more understandable.
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/extents.c

@@ -553,7 +553,7 @@ static bool bch_extent_bad(struct btree_keys *bk, const struct bkey *k)
 	for (i = 0; i < KEY_PTRS(k); i++) {
 		stale = ptr_stale(b->c, k, i);
 
-		btree_bug_on(stale > 96, b,
+		btree_bug_on(stale > BUCKET_GC_GEN_MAX, b,
 			     "key too stale: %i, need_gc %u",
 			     stale, b->c->need_gc);