virt: sev-guest: Remove is_vmpck_empty() helper

virt: sev-guest: Remove is_vmpck_empty() helper

Remove is_vmpck_empty() which uses a local array allocation to check if the
VMPCK is empty and replace it with memchr_inv() to directly determine if the
VMPCK is empty without additional memory allocation.

  [ bp: Massage commit message. ]

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250106124633.1418972-2-nikunj@amd.com