Add an accessor -- ufs_maxphys() -- to check the maximum transfer size
for a given UFS mountpoint, and move the code from mount that finds
the underlying disk and resets the mountpoint max transfer size into a
utility function, ufs_update_maxphys().

Add a global serial number that counts disk property changes to which
filesystems are meant to accomodate themselves.  Make ufs_maxphys()
check it.  This is a sort of flag-polling interface that avoids callbacks
into the filesystem code, but will require freezing filesystems and
draining in-flight transactions before a decrease in size that is
mandatory (like attaching a disk with a smaller maximum transfer size
as a spare in a RAIDframe set), rather than "advisory", like finding
out set geometry from a RAID controller long after boot and deciding
a smaller transfer size would be optimal, can be signalled.  Still, the
"advisory" case is the common one so this is progress.

Make a bit of an example of RAIDframe by making it bump this new
serial number when disks are added to the subsystem.  I will attack
one of the hardware RAID drivers (probably arcmsr) next.