ext4: use do_div() to calculate block offset

Use do_div() to calculate the block offset and the offset
within the block, make the code more concise.

Signed-off-by: Chunguang Xu <brookxu@tencent.com>

fs/ext4/balloc.c

@@ -265,7 +265,7 @@ struct ext4_group_desc * ext4_get_group_desc(struct super_block *sb,
 					     ext4_group_t block_group,
 					     struct buffer_head **bh)
 {
-	unsigned int group_desc;
+	unsigned int group_desc = block_group;
 	unsigned int offset;
 	ext4_group_t ngroups = ext4_get_groups_count(sb);
 	struct ext4_group_desc *desc;
@@ -279,8 +279,7 @@ struct ext4_group_desc * ext4_get_group_desc(struct super_block *sb,
 		return NULL;
 	}
 
-	group_desc = block_group >> EXT4_DESC_PER_BLOCK_BITS(sb);
-	offset = block_group & (EXT4_DESC_PER_BLOCK(sb) - 1);
+	offset = do_div(group_desc, EXT4_DESC_PER_BLOCK(sb));
 	bh_p = sbi_array_rcu_deref(sbi, s_group_desc, group_desc);
 	/*
 	 * sbi_array_rcu_deref returns with rcu unlocked, this is ok since