pr_debug

enable pr_debug in bcache

1. mount -t debugfs /mnt/debugfs/ if debugfs is not mounted in /sys/kernel/debug/
2. echo 'file btree.c line 2349 +p' > /mnt/debugfs/dynamic_debug/control #show pr_debug in btree.c line 2349
3. echo 'module bcache +p' > /mnt/debugfs/dynamic_debug/control #show all pr_debug in bcache
4. "grep bch_btree_refill_keybuf /var/log/debug" or "dmesg|grep bch_btree_refill_keybuf"