btrfs: handle NULL roots in btrfs_put/btrfs_grab_fs_root

We want to use this for dropping all roots, and in some error cases we
may not have a root, so handle this to make the cleanup code easier.
Make btrfs_grab_fs_root the same so we can use it in cases where the
root may not exist (like the quota root).
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.h

@@ -88,6 +88,8 @@ struct btrfs_root *btrfs_alloc_dummy_root(struct btrfs_fs_info *fs_info);
  */
 static inline struct btrfs_root *btrfs_grab_fs_root(struct btrfs_root *root)
 {
+	if (!root)
+		return NULL;
 	if (refcount_inc_not_zero(&root->refs))
 		return root;
 	return NULL;
@@ -95,6 +97,8 @@ static inline struct btrfs_root *btrfs_grab_fs_root(struct btrfs_root *root)
 
 static inline void btrfs_put_fs_root(struct btrfs_root *root)
 {
+	if (!root)
+		return;
 	if (refcount_dec_and_test(&root->refs))
 		kfree(root);
 }