x86/sev: Fix address space sparse warning

Fix:

  arch/x86/kernel/sev.c:605:16: warning: incorrect type in assignment (different address spaces)
  arch/x86/kernel/sev.c:605:16:    expected struct snp_secrets_page_layout *layout
  arch/x86/kernel/sev.c:605:16:    got void [noderef] __iomem *[assigned] mem

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/202205022233.XgNDR7WR-lkp@intel.com