BACKPORT: FROMGIT: KVM: arm64: Introduce kvm_share_hyp()

The create_hyp_mappings() function can currently be called at any point
in time. However, its behaviour in protected mode changes widely
depending on when it is being called. Prior to KVM init, it is used to
create the temporary page-table used to bring-up the hypervisor, and
later on it is transparently turned into a 'share' hypercall when the
kernel has lost control over the hypervisor stage-1. In order to prepare
the ground for also unsharing pages with the hypervisor during guest
teardown, introduce a kvm_share_hyp() function to make it clear in which
places a share hypercall should be expected, as we will soon need a
matching unshare hypercall in all those places.

[ qperret: BACKPORT due to resolution in kvm_arch_vcpu_run_map_fp() in
  merge commit 2d761dbf ("Merge branch kvm-arm64/fpsimd-tracking
  into kvmarm-master/next") ]

Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211215161232.1480836-7-qperret@google.com
(cherry picked from commit 3f868e14
 git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git next)
Bug: 209599700
Change-Id: Id3ba033a51550e5aada862d6af06773d0be833fb