Properly handle possible underflow in vm_fault_prefault().

In vm_fault_prefault(), if backward count causes underflow in
calculation of
starta = addra - backward * PAGE_SIZE;
then starta must be clipped to entry->start, instead of zero.
Clipping to zero allowed mapping outside of the map entries address
ranges, in particular, map at zero.

Submitted by: Yanko Yankulov <yanko.yankulov@gmail.com>
Reviewed by: alc
MFC after: 1 week