btrfs: Add trace for btrfs_workqueue alloc/destroy

Since most of the btrfs_workqueue is printed as pointer address,
for easier analysis, add trace for btrfs_workqueue alloc/destroy.
So it is possible to determine the workqueue that a given work belongs
to(by comparing the wq pointer address with alloc trace event).

Signed-off-by: Qu Wenruo <quenruo@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 0a891cd..9c6b66d1 100644
--- a/fs/btrfs/async-thread.h
+++ b/fs/btrfs/async-thread.h
@@ -38,7 +38,7 @@
 	unsigned long flags;
 };
 
-struct btrfs_workqueue *btrfs_alloc_workqueue(char *name,
+struct btrfs_workqueue *btrfs_alloc_workqueue(const char *name,
 					      int flags,
 					      int max_active,
 					      int thresh);