Apply the limit to avoid the overflows in the radix tree subr_blist.c
after the conversion of the swap device size to the page size units,
not before. That lifts the limit on the usable swap partition size
from 32GB to 256GB, that is less depressing for the modern systems.

Submitted by: Alexander V. Chernikov <melifaro ipfw ru>
Reviewed by: alc
Approved by: re (bz)
MFC after: 2 weeks