ext2: code cleanup by calling ext2_group_last_block_no()

Call common helper ext2_group_last_block_no() to
calculate group last block number.

Link: https://lore.kernel.org/r/20191104114036.9893-2-cgxu519@mykernel.net


Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Jan Kara <jack@suse.cz>