btrfs: use slab for end_io_wq structures

The structure is frequently reused.  Rename it according to the slab
name.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index b915d77..4685b97 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2001,10 +2001,14 @@
 	if (err)
 		goto free_delayed_ref;
 
-	err = btrfs_interface_init();
+	err = btrfs_end_io_wq_init();
 	if (err)
 		goto free_prelim_ref;
 
+	err = btrfs_interface_init();
+	if (err)
+		goto free_end_io_wq;
+
 	btrfs_init_lockdep();
 
 	btrfs_print_info();
@@ -2021,6 +2025,8 @@
 
 unregister_ioctl:
 	btrfs_interface_exit();
+free_end_io_wq:
+	btrfs_end_io_wq_exit();
 free_prelim_ref:
 	btrfs_prelim_ref_exit();
 free_delayed_ref: