selftests/mm: Use PKEY_UNRESTRICTED macro

Replace literal 0 with macro PKEY_UNRESTRICTED where pkey_*() functions
are used in mm selftests for memory protection keys.

Signed-off-by: Yury Khrustalev <yury.khrustalev@arm.com>
Suggested-by: Joey Gouly <joey.gouly@arm.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/r/20250113170619.484698-3-yury.khrustalev@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>