KVM: nVMX: Use vmx_segment_cache_clear() instead of open coded equivalent

KVM: nVMX: Use vmx_segment_cache_clear() instead of open coded equivalent

In prepare_vmcs02_rare(), call vmx_segment_cache_clear() instead of
setting segment_cache.bitmask directly.  Using the helper minimizes the
chances of prepare_vmcs02_rare() doing the wrong thing in the future, e.g.
if KVM ends up doing more than just zero the bitmask when purging the
cache.

No functional change intended.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20240725175232.337266-2-mlevitsk@redhat.com
[sean: massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>