btrfs: Don't BUG_ON alloc_path errors in btrfs_truncate_inode_items

I moved the path allocation up a few lines to the top of the function so
that we couldn't get into the state where we've dropped delayed items and
the extent cache but fail due to -ENOMEM.
Signed-off-by: Mark Fasheh <mfasheh@suse.com>

fs/btrfs/inode.c

@@ -3172,6 +3172,11 @@ int btrfs_truncate_inode_items(struct btrfs_trans_handle *trans,
 
 	BUG_ON(new_size > 0 && min_type != BTRFS_EXTENT_DATA_KEY);
 
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+	path->reada = -1;
+
 	if (root->ref_cows || root == root->fs_info->tree_root)
 		btrfs_drop_extent_cache(inode, new_size & (~mask), (u64)-1, 0);
 
@@ -3184,10 +3189,6 @@ int btrfs_truncate_inode_items(struct btrfs_trans_handle *trans,
 	if (min_type == 0 && root == BTRFS_I(inode)->root)
 		btrfs_kill_delayed_inode_items(inode);
 
-	path = btrfs_alloc_path();
-	BUG_ON(!path);
-	path->reada = -1;
-
 	key.objectid = ino;
 	key.offset = (u64)-1;
 	key.type = (u8)-1;