arm64/mm: Define dummy pud_user_exec() when using 2-level page-table

With only two levels of page-table, the generic 'pud_*' macros are
implemented using dummy operations in pgtable-nopmd.h. Since commit
730a11f982 ("arm64/mm: add pud_user_exec() check in
pud_user_accessible_page()"), pud_user_accessible_page() unconditionally
calls pud_user_exec(), which is an arm64-specific helper and therefore
isn't defined by pgtable-nopmd.h. This results in a build failure for
configurations with only two levels of page table:

   arch/arm64/include/asm/pgtable.h: In function 'pud_user_accessible_page':
>> arch/arm64/include/asm/pgtable.h:870:51: error: implicit declaration of function 'pud_user_exec'; did you mean 'pmd_user_exec'? [-Werror=implicit-function-declaration]
     870 |         return pud_leaf(pud) && (pud_user(pud) || pud_user_exec(pud));
         |                                                   ^~~~~~~~~~~~~
         |                                                   pmd_user_exec

Fix the problem by defining pud_user_exec() as pud_user() in this case.

Link: https://lore.kernel.org/r/202301080515.z6zEksU4-lkp@intel.com
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/pgtable.h

@@ -730,6 +730,7 @@ static inline pmd_t *pud_pgtable(pud_t pud)
 #else
 
 #define pud_page_paddr(pud)	({ BUILD_BUG(); 0; })
+#define pud_user_exec(pud)	pud_user(pud) /* Always 0 with folding */
 
 /* Match pmd_offset folding in <asm/generic/pgtable-nopmd.h> */
 #define pmd_set_fixmap(addr)		NULL