mkfs: don't pass on extent size inherit flags when extent size is zero

mkfs: don't pass on extent size inherit flags when extent size is zero

If the caller passes in an extent size hint of zero, clear the inherit
flags because a hint value of zero is treated as not a hint.

Otherwise, you get stupid stuff like:
$ mkfs.xfs -d cowextsize=0 /tmp/a.img -f
illegal CoW extent size hint 0, must be less than 9600.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>