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 <[e-mail]>

fs/btrfs/ioctl.c

···
 
 	pending_snapshot->root_item = kzalloc(sizeof(struct btrfs_root_item),
 			GFP_NOFS);
-	if (!pending_snapshot->root_item) {
+	pending_snapshot->path = btrfs_alloc_path();
+	if (!pending_snapshot->root_item || !pending_snapshot->path) {
 		ret = -ENOMEM;
 		goto free_pending;
 	}
···
 		wake_up_atomic_t(&root->will_be_snapshoted);
 free_pending:
 	kfree(pending_snapshot->root_item);
+	btrfs_free_path(pending_snapshot->path);
 	kfree(pending_snapshot);
 
 	return ret;

fs/btrfs/transaction.c

···
 	u64 root_flags;
 	uuid_le new_uuid;
 
-	path = btrfs_alloc_path();
-	if (!path) {
-		pending->error = -ENOMEM;
-		return 0;
-	}
+	ASSERT(pending->path);
+	path = pending->path;
 
 	ASSERT(pending->root_item);
 	new_root_item = pending->root_item;
···
 	kfree(new_root_item);
 	pending->root_item = NULL;
 	btrfs_free_path(path);
+	pending->path = NULL;
+
 	return ret;
 }
 

fs/btrfs/transaction.h

···
 	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;