mkfs.xfs.8: Fix an inconsistency between the code and the man page.

mkfs.xfs.8: Fix an inconsistency between the code and the man page.

The man page currently states that block and sector size units cannot
be used for other option values unless they are explicitly specified,
when in fact the default sizes will be used in that case.

Change the man page to clarify this.

Signed-off-by: Alvin Zheng <Alvin@linux.alibaba.com>
[sandeen: sector/block values do not need to be specified first]
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>