arm64: mm: dump: fix shift warning

When building with 48-bit VAs, it's possible to get the following
warning when building the arm64 page table dumping code:

arch/arm64/mm/dump.c: In function ‘walk_pgd’:
arch/arm64/mm/dump.c:266:2: warning: right shift count >= width of type
  pgd_t *pgd = pgd_offset(mm, 0);
  ^

As pgd_offset is a macro and the second argument is not cast to any
particular type, the zero will be given integer type by the compiler.
As pgd_offset passes the pargument to pgd_index, we then try to shift
the 32-bit integer by at least 39 bits (for 4k pages).

Elsewhere the pgd_offset is passed a second argument of unsigned long
type, so let's do the same here by passing '0UL' rather than '0'.

Cc: Kees Cook <keescook@chromium.org>
Acked-by: NLaura Abbott <lauraa@codeaurora.org>
Acked-by: NSteve Capper <steve.capper@arm.com>
Signed-off-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/mm/dump.c

@@ -272,7 +272,7 @@ static void walk_pud(struct pg_state *st, pgd_t *pgd, unsigned long start)
 
 static void walk_pgd(struct pg_state *st, struct mm_struct *mm, unsigned long start)
 {
-	pgd_t *pgd = pgd_offset(mm, 0);
+	pgd_t *pgd = pgd_offset(mm, 0UL);
 	unsigned i;
 	unsigned long addr;