KVM: arm64: Remove VM-wide VNCR mapping counter

In the Linux kernel, the following vulnerability has been resolved:

KVM: arm64: Remove VM-wide VNCR mapping counter

The global VNCR mapping counter is used to decide whether an L1
provided VNCR page is mapped in L0 on any CPU at the point of
dealing with a TLB invalidation. It is incremented when a mapping
is made in the fixmap, and decremented when unmapped.

As it turns out, this tracking has several flaws:

- we are trying to invalidate TLBs, and the mapping is only an
opportunistic consequence of the TLB. Checking this counter to
decide whether a TLB needs to be invalidated may result in missed
invalidations.

- an L1 vcpu invalidating its own TLB (a very likely case) will not
succeed in invalidating the VNCR pseudo TLB because that page is
not mapped in L0 at this stage.

Given that this tracking fails at delivering the minimum guarantees
that are required and is only a performance optimisation, remove it
completely.