btrfs: drop null testing before destroy functions

Cleanup.

kmem_cache_destroy has support NULL argument checking,
so drop the double null testing before calling it.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c
index 0be47e4..716a4ec 100644
--- a/fs/btrfs/delayed-inode.c
+++ b/fs/btrfs/delayed-inode.c
@@ -43,8 +43,7 @@
 
 void btrfs_delayed_inode_exit(void)
 {
-	if (delayed_node_cache)
-		kmem_cache_destroy(delayed_node_cache);
+	kmem_cache_destroy(delayed_node_cache);
 }
 
 static inline void btrfs_init_delayed_node(