Make grab_valid more consistent with other grab functions by requiring ZERO.

[jeff]

Summary

After this change vm_page_grab_valid() requires callers to specify ZERO if they want to fill in sparse files/objects. I audited all consumers and a couple wanted it and a couple did not. This makes valid much more consistent with the others although there are still quirks around NOWAIT/WAITFAIL that could be eliminated if necessary.

Test Plan

stress2

[kib]

I think that the behavior of VM_ALLOC_ZERO/has_page should be documented in the comment.