Btrfs: send, don't leave without decrementing clone root's send_progress

If the clone root was not readonly or the dead flag was set on it, we were
leaving without decrementing the root's send_progress counter (and before
we just incremented it). If a concurrent snapshot deletion was in progress
and ended up being aborted, it would be impossible to later attempt to
delete again the snapshot, since the root's send_in_progress counter could
never go back to 0.

We were also setting clone_sources_to_rollback to i + 1 too early - if we
bailed out because the clone root we got is not readonly or flagged as dead
we ended up later derreferencing a null pointer because we didn't assign
the clone root to sctx->clone_roots[i].root:

		for (i = 0; sctx && i < clone_sources_to_rollback; i++)
			btrfs_root_dec_send_in_progress(
					sctx->clone_roots[i].root);

So just don't increment the send_in_progress counter if the root is readonly
or flagged as dead.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/send.c

@@ -5852,9 +5852,7 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
 				ret = PTR_ERR(clone_root);
 				goto out;
 			}
-			clone_sources_to_rollback = i + 1;
 			spin_lock(&clone_root->root_item_lock);
-			clone_root->send_in_progress++;
 			if (!btrfs_root_readonly(clone_root) ||
 			    btrfs_root_dead(clone_root)) {
 				spin_unlock(&clone_root->root_item_lock);
@@ -5862,10 +5860,12 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
 				ret = -EPERM;
 				goto out;
 			}
+			clone_root->send_in_progress++;
 			spin_unlock(&clone_root->root_item_lock);
 			srcu_read_unlock(&fs_info->subvol_srcu, index);
 
 			sctx->clone_roots[i].root = clone_root;
+			clone_sources_to_rollback = i + 1;
 		}
 		vfree(clone_sources_tmp);
 		clone_sources_tmp = NULL;