After r118390, the variable "dmmax" was neither the correct strip size
nor the correct maximum block size. Moreover, after r318995, it serves
no purpose except to provide information to user space through a read-
sysctl.

This change eliminates the variable "dmmax" but retains the sysctl. It
also corrects the value returned by the sysctl.

Reviewed by: kib, markj
MFC after: 3 days