btrfs: preallocate path for snapshot creation at ioctl time

We can also preallocate btrfs_path that's used during pending snapshot
creation and avoid another late ENOMEM failure.

Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h
index b6f9a3c..72be51f 100644
--- a/fs/btrfs/transaction.h
+++ b/fs/btrfs/transaction.h
@@ -140,6 +140,7 @@
 	struct btrfs_root_item *root_item;
 	struct btrfs_root *snap;
 	struct btrfs_qgroup_inherit *inherit;
+	struct btrfs_path *path;
 	/* block reservation for the operation */
 	struct btrfs_block_rsv block_rsv;
 	u64 qgroup_reserved;