bcache: reduce gc latency by processing less nodes and sleep less time

bcache: reduce gc latency by processing less nodes and sleep less time

When bcache device is busy for high I/O loads, there are two methods to
reduce the garbage collection latency,
- Process less nodes in eac loop of incremental garbage collection in
  btree_gc_recurse().
- Sleep less time between two full garbage collection in
  bch_btree_gc().

This patch introduces to hleper routines to provide different garbage
collection nodes number and sleep intervel time.
- btree_gc_min_nodes()
  If there is no front end I/O, return 128 nodes to process in each
  incremental loop, otherwise only 10 nodes are returned. Then front I/O
  is able to access the btree earlier.
- btree_gc_sleep_ms()
  If there is no synchronized wait for bucket allocation, sleep 100 ms
  between two incremental GC loop. Othersize only sleep 10 ms before
  incremental GC loop. Then a faster GC may provide available buckets
  earlier, to avoid most of bcache working threads from being starved by
  buckets allocation.

The idea is inspired by works from Mingzhe Zou and Robert Pang, but much
simpler and the expected behavior is more predictable.

Signed-off-by: Coly Li <colyli@fnnas.com>
Signed-off-by: Robert Pang <robertpang@google.com>
Signed-off-by: Mingzhe Zou <mingzhe.zou@easystack.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>