Hide all vm/vm_pageout.h content under #ifdef _KERNEL.

There are no parts useful for usermode applications in
vm/vm_pageout.h. Even for the specific applications like fstat and
lsof.

In my opinion, this protection is redundant and instead userspace
should not include the header at all. Since there are apparently
broken third party codebases, give them a bit of slack by providing
transitional period.

Reported by: julian
Sponsored by: The FreeBSD Foundation
MFC after: 1 week