m68k/mm/motorola: specify pmd_page() type

Failing to specify a specific type here breaks anything that relies on the
type being explicitly known, such as page_folio().

Make explicit the type of null pointer returned here.

Link: https://lkml.kernel.org/r/ad6be2821bbd6af10966b3704568ff458b270d9c.1673526881.git.lstoakes@gmail.com
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/m68k/include/asm/motorola_pgtable.h

@@ -124,7 +124,7 @@ static inline void pud_set(pud_t *pudp, pmd_t *pmdp)
  * expects pmd_page() to exists, only to then DCE it all. Provide a dummy to
  * make the compiler happy.
  */
-#define pmd_page(pmd)		NULL
+#define pmd_page(pmd)		((struct page *)NULL)
 
 
 #define pud_none(pud)		(!pud_val(pud))