In r118390, the swap pager's approach to striping swap allocation over
multiple devices was changed. However, swapoff_one() was not fully and
correctly converted. In particular, with r118390's introduction of a per-
device blist, the maximum swap block size, "dmmax", became irrelevant to
swapoff_one()'s operation. Moreover, swapoff_one() was performing out-of-
range operations on the per-device blist that were silently ignored by
blist_fill().

This change corrects both of these problems with swapoff_one(), which will
allow us to potentially increase MAX_PAGEOUT_CLUSTER. Previously,
swapoff_one() would panic inside of blist_fill() if you increased
MAX_PAGEOUT_CLUSTER.

Reviewed by: kib, markj
MFC after: 3 days