K 10
svn:author
V 3
kib
K 8
svn:date
V 27
2017-02-24T08:09:16.577370Z
K 7
svn:log
V 435
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

END