arm64/mm: Fix pud_user_accessible_page() for PGTABLE_LEVELS <= 2

The recent change to use pud_valid() as part of the implementation of
pud_user_accessible_page() fails to build when PGTABLE_LEVELS <= 2
because pud_valid() is not defined in that case.

Fix this by defining pud_valid() to false for this case. This means that
pud_user_accessible_page() will correctly always return false for this
config.

Fixes: f0f5863a ("arm64/mm: Remove PTE_PROT_NONE bit")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202405082221.43rfWxz5-lkp@intel.com/Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20240509122844.563320-1-ryan.roberts@arm.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/pgtable.h

@@ -814,6 +814,7 @@ static inline pmd_t *pud_pgtable(pud_t pud)
 
 #else
 
+#define pud_valid(pud)		false
 #define pud_page_paddr(pud)	({ BUILD_BUG(); 0; })
 #define pud_user_exec(pud)	pud_user(pud) /* Always 0 with folding */