Adding zfs_min_auto_ashift any objections?

[Will Andrews]

From: Steven Hartland smh at freebsd.org
I'm looking to add the ability to configure a system wide  
minimum ashift value which is useful for future compatibility  
e.g. when you want to force the use of 4k sectors even though  
current disks are 512b.  

Now we have logical / physical sector size calculations in the  
tree this makes doing this trivial but wanted to check no one  
has any objections to the attached patch?  

A few nits:
- Please reduce the line length to < 80 chars.
- style(9): indent followup lines by 4 spaces, not tabs.

—Will.