ext4: fix unused iterator variable warnings

ext4: fix unused iterator variable warnings

When CONFIG_QUOTA is disabled, there are warnings around unused iterator
variables:

  fs/ext4/super.c: In function 'ext4_put_super':
  fs/ext4/super.c:1262:13: error: unused variable 'i' [-Werror=unused-variable]
   1262 |         int i, err;
        |             ^
  fs/ext4/super.c: In function '__ext4_fill_super':
  fs/ext4/super.c:5200:22: error: unused variable 'i' [-Werror=unused-variable]
   5200 |         unsigned int i;
        |                      ^
  cc1: all warnings being treated as errors

The kernel has updated to GNU11, allowing the variables to be declared
within the for loop.  Do so to clear up the warnings.

Fixes: dcbf87589d90 ("ext4: factor out ext4_flex_groups_free()")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20230420-ext4-unused-variables-super-c-v1-1-138b6db6c21c@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>