Correctly handle misaligned VMIO buffer (whose start or end offset in the VM
object are not page aligned). This should fix the mount_msdos panic after a
failed attemp to mount as ffs.
Reviewed By: Matthew Dillon <dillon@apollo.backplane.com>
Archie Cobbs <archie@whistle.com> Dmitrij Tejblum <dima@tejblum.dnttm.rssi.ru>