ext2: introduce new helper ext2_group_last_block_no()

Introduce new helper ext2_group_last_block_no() to calculate
last block num for specific block group, we can replace open
coded logic by calling this common helper.

Link: https://lore.kernel.org/r/20191104114036.9893-1-cgxu519@mykernel.net


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