Btrfs: allocate prelim_ref with a slab allocater

struct __prelim_ref is allocated and freed frequently when
walking backref tree, using slab allocater can not only
speed up allocating but also detect memory leaks.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Reviewed-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 4d5d0f3..3aab10c 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -56,6 +56,7 @@
 #include "rcu-string.h"
 #include "dev-replace.h"
 #include "free-space-cache.h"
+#include "backref.h"
 #include "tests/btrfs-tests.h"
 
 #define CREATE_TRACE_POINTS
@@ -1810,6 +1811,10 @@
 	if (err)
 		goto free_auto_defrag;
 
+	err = btrfs_prelim_ref_init();
+	if (err)
+		goto free_prelim_ref;
+
 	err = btrfs_interface_init();
 	if (err)
 		goto free_delayed_ref;
@@ -1830,6 +1835,8 @@
 
 unregister_ioctl:
 	btrfs_interface_exit();
+free_prelim_ref:
+	btrfs_prelim_ref_exit();
 free_delayed_ref:
 	btrfs_delayed_ref_exit();
 free_auto_defrag:
@@ -1856,6 +1863,7 @@
 	btrfs_delayed_ref_exit();
 	btrfs_auto_defrag_exit();
 	btrfs_delayed_inode_exit();
+	btrfs_prelim_ref_exit();
 	ordered_data_exit();
 	extent_map_exit();
 	extent_io_exit();