x86/hyperv: Fix definition of hv_ghcb_pg variable

The percpu variable hv_ghcb_pg is incorrectly defined.  The __percpu
qualifier should be associated with the union hv_ghcb * (i.e.,
a pointer), not with the target of the pointer. This distinction
makes no difference to gcc and the generated code, but sparse
correctly complains.  Fix the definition in the interest of
general correctness in addition to making sparse happy.

No functional change.

Fixes: 0cc4f6d9

 ("x86/hyperv: Initialize GHCB page in Isolation VM")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1640662315-22260-2-git-send-email-mikelley@microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>