mkfs: Remove workaround for getsubopt() on older glibc

mkfs: Remove workaround for getsubopt() on older glibc

The workaround addressed a const-correctness warning on glibc
versions older than 2.2. However, it also captures alternative C
libraries on Linux which it should not do. glibc is really old, so
let's just remove the workaround.

Signed-off-by: Felix Janda <felix.janda@posteo.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>