btrfs: async_thread: Fix workqueue 'max_active' value when initializing

At initializing time, for threshold-able workqueue, it's max_active
of kernel workqueue should be 1 and grow if it hits threshold.

But due to the bad naming, there is both 'max_active' for kernel
workqueue and btrfs workqueue.
So wrong value is given at workqueue initialization.

This patch fixes it, and to avoid further misunderstanding, change the
member name of btrfs_workqueue to 'current_active' and 'limit_active'.

Also corresponding comment is added for readability.

Reported-by: Alex Lyakas <alex.btrfs@zadarastorage.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/async-thread.h b/fs/btrfs/async-thread.h
index b0b093b..ad4d064 100644
--- a/fs/btrfs/async-thread.h
+++ b/fs/btrfs/async-thread.h
@@ -69,7 +69,7 @@
 
 struct btrfs_workqueue *btrfs_alloc_workqueue(const char *name,
 					      unsigned int flags,
-					      int max_active,
+					      int limit_active,
 					      int thresh);
 void btrfs_init_work(struct btrfs_work *work, btrfs_work_func_t helper,
 		     btrfs_func_t func,