xfs: rename `new' to `delta' in xfs_growfs_data_private()

It actually means the delta block count of growfs. Rename it in order
to make it clear. Also introduce nb_div to avoid reusing `delta`.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>