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/file.c b/fs/btrfs/file.c
index 610f569..b387fb5 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2939,8 +2939,7 @@
 
 void btrfs_auto_defrag_exit(void)
 {
-	if (btrfs_inode_defrag_cachep)
-		kmem_cache_destroy(btrfs_inode_defrag_cachep);
+	kmem_cache_destroy(btrfs_inode_defrag_cachep);
 }
 
 int btrfs_auto_defrag_init(void)