ext4: Fix initalization of s_flex_groups

The s_flex_groups array should have been initialized using atomic_add
to sum up the free counts from the block groups that make up a
flex_bg.  By using atomic_set, the value of the s_flex_groups array
was set to the values of the last block group in the flex_bg.  

The impact of this bug is that the block and inode allocation
algorithms might not pick the best flex_bg for new allocation.

Thanks to Damien Guibouret for pointing out this problem!
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -1694,12 +1694,12 @@ static int ext4_fill_flex_info(struct super_block *sb)
 		gdp = ext4_get_group_desc(sb, i, NULL);
 
 		flex_group = ext4_flex_group(sbi, i);
-		atomic_set(&sbi->s_flex_groups[flex_group].free_inodes,
-			   ext4_free_inodes_count(sb, gdp));
-		atomic_set(&sbi->s_flex_groups[flex_group].free_blocks,
-			   ext4_free_blks_count(sb, gdp));
-		atomic_set(&sbi->s_flex_groups[flex_group].used_dirs,
-			   ext4_used_dirs_count(sb, gdp));
+		atomic_add(ext4_free_inodes_count(sb, gdp),
+			   &sbi->s_flex_groups[flex_group].free_inodes);
+		atomic_add(ext4_free_blks_count(sb, gdp),
+			   &sbi->s_flex_groups[flex_group].free_blocks);
+		atomic_add(ext4_used_dirs_count(sb, gdp),
+			   &sbi->s_flex_groups[flex_group].used_dirs);
 	}
 
 	return 1;