[NTOSKRNL] Only consider SharedCacheMap value once ViewLock is acquired.

[NTOSKRNL] Only consider SharedCacheMap value once ViewLock is acquired.

This avoids a really nasty race condition in our cache controler where
two concurrents could try to initialize cache on the same file.
This had two nasty effects: first shared map was purely leaked and erased
by the second one. And the private cache map, allocated on the first shared
cache map couldn't be freed and was leading to Mm BSOD (free in a middle of
a block).

This was often triggered while building ReactOS on ReactOS (with multi threads).
With that patch, I cannot crash anylonger while building ReactOS.

CORE-14634