mkfs.xfs.8: Clarify mkfs vs. mount block size limits.

mkfs.xfs.8: Clarify mkfs vs. mount block size limits.

The current explanation is a bit vague to the uninitiated;
clarify the text to make it obvious that while the mkfs.xfs
utility is able to create valid filesystems with block sizes
up to 65536, the Linux kernel can only mount filesystems with
page sized or smaller blocks.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>