When creating a new FFS file system, the block size will indirectly

When creating a new FFS file system, the block size will indirectly
affect the largest file size that is allowed by the file system.
On the other hand, when creating a snapshot, the snapshot file will
appear as it is as big as the file system itself.  Hence we will not
be able to create a snapshot on large file systems with small block
sizes.

Add a warning about this, and gives some hints to correct the issue.

Reviewed by:    mckusick
MFC After:      1 week