ext4: simplify calculation of blkoff in ext4_mb_new_blocks_simple

We try to allocate a block from goal in ext4_mb_new_blocks_simple. We
only need get blkoff in first group with goal and set blkoff to 0 for
the rest groups.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://lore.kernel.org/r/20230303172120.3800725-21-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>