DF-2676 fix validation on patched kernel #1 (both DF-2676+DF-2677 fixes applied):

$ cd /root/df2676_mod && make && kldload ./df2676.ko
cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv -r -Wl,-d -o df2676.ko df2676.o
KLDLOAD_RC=0
GUEST_STILL_UP
DF-2676: loaded, page 0xfffff80049b20000 busied in obj 0xfffff80116ccd100
DF-2676: grabber: calling vm_page_grab(obj, 0, VM_ALLOC_NORMAL) - NO VM_ALLOC_RETRY - on busy page
DF-2676: holder: waking busy page 0xfffff80049b20000
DF-2676: grabber: grab returned 0 (no panic?!)
(then: dmesg confirms the four lines above; guest continued running -
 it only went down ~4 minutes later from the separate DF-2677 race, whose
 fix is incomplete - see ../DF-2677/panic_postfix.txt)

BASELINE (stock kernel, same module):
DF-2676: grabber: calling vm_page_grab(obj, 0, VM_ALLOC_NORMAL) - NO VM_ALLOC_RETRY - on busy page
DF-2676: holder: waking busy page 0xfffff80049701200
Fatal trap 12: page fault while in kernel mode
fault virtual address = 0x76
Stopped at vm_page_grab.cold.30: movzbl 0x76,%eax        (panic.txt)