vm_page_is_valid() wasn't expecting a large offset argument, it's

vm_page_is_valid() wasn't expecting a large offset argument, it's
expecting a sub-page offset.  We were passing the file position,
and vm_page_bits() could do some interesting things when base was
larger PAGE_SIZE.
if (size > PAGE_SIZE - base)
	size = PAGE_SIZE - base;
is interesting when (PAGE_SIZE - base) is negative.  I could imagine that
this could have interesting consequences for memory page -> device block
bit validation.