Btrfs: don't BUG_ON() in btrfs_truncate_inode_items()

btrfs_free_extent() can fail because of ENOMEM. There's no reason to
panic here, we can just abort the transaction.

Fixes: f4b9aa8d3b87 ("btrfs_truncate")
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -4918,7 +4918,10 @@ delete:
 						extent_num_bytes, 0,
 						btrfs_header_owner(leaf),
 						ino, extent_offset);
-			BUG_ON(ret);
+			if (ret) {
+				btrfs_abort_transaction(trans, ret);
+				break;
+			}
 			if (btrfs_should_throttle_delayed_refs(trans, fs_info))
 				btrfs_async_run_delayed_refs(fs_info,
 					trans->delayed_ref_updates * 2,