mkfs: Break block discard into chunks of 2 GB

mkfs: Break block discard into chunks of 2 GB

Some users are not happy about the BLKDISCARD taking too long and at the
same time not being informed about that - so they think that the command
actually hung.

This commit changes code so that progress reporting is possible and also
typing the ^C will cancel the ongoing BLKDISCARD.

Signed-off-by: Pavel Reichl <preichl@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>