bcache: allow allocator to invalidate bucket in gc

Currently, if the gc is running, when the allocator found free_inc
is empty, allocator has to wait the gc finish. Before that, the
IO is blocked.

But actually, there would be some buckets is reclaimable before gc,
and gc will never mark this kind of bucket to be unreclaimable.

So we can put these buckets into free_inc in gc running to avoid
IO being blocked.

Signed-off-by: Dongsheng Yang <dongsheng.yang@easystack.cn>
Signed-off-by: Mingzhe Zou <mingzhe.zou@easystack.cn>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20240528120914.28705-2-colyli@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>