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/disk-io.c b/fs/btrfs/disk-io.c
index 4545e2e..19f8e5b 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -110,8 +110,7 @@
 
 void btrfs_end_io_wq_exit(void)
 {
-	if (btrfs_end_io_wq_cache)
-		kmem_cache_destroy(btrfs_end_io_wq_cache);
+	kmem_cache_destroy(btrfs_end_io_wq_cache);
 }
 
 /*