mm/debug_vm_pgtable: Remove uninitialized_var() usage

Using uninitialized_var() is dangerous as it papers over real bugs[1]
(or can in the future), and suppresses unrelated compiler warnings (e.g.
"unused variable"). If the compiler thinks it is uninitialized, either
simply initialize the variable or make compiler changes. As a precursor
to removing[2] this[3] macro[4], just initialize this variable to NULL.

[1] https://lore.kernel.org/lkml/[e-mail]/
[2] https://lore.kernel.org/lkml/CA+55aFw+Vbj0i=[e-mail]/
[3] https://lore.kernel.org/lkml/[e-mail]/
[4] https://lore.kernel.org/lkml/CA+55aFz2500WfbKXAx8s67wrm9=[e-mail]/

Fixes: 399145f9eb6c ("mm/debug: add tests validating architecture page table helpers")
Reviewed-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>

mm/debug_vm_pgtable.c

···
 	phys_addr_t paddr;
 	unsigned long vaddr, pte_aligned, pmd_aligned;
 	unsigned long pud_aligned, p4d_aligned, pgd_aligned;
-	spinlock_t *uninitialized_var(ptl);
+	spinlock_t *ptl = NULL;
 
 	pr_info("Validating architecture page table helpers\n");
 	prot = vm_get_page_prot(VMFLAGS);