m68k: fix "bad page state" oops on ColdFire boot

Booting a ColdFire m68k core with MMU enabled causes a "bad page state"
oops since commit 1d40a5ea ("mm: mark pages in use for page tables"):

 BUG: Bad page state in process sh  pfn:01ce2
 page:004fefc8 count:0 mapcount:-1024 mapping:00000000 index:0x0
 flags: 0x0()
 raw: 00000000 00000000 00000000 fffffbff 00000000 00000100 00000200 00000000
 raw: 039c4000
 page dumped because: nonzero mapcount
 Modules linked in:
 CPU: 0 PID: 22 Comm: sh Not tainted 4.17.0-07461-g1d40a5ea #13

Fix by calling pgtable_page_dtor() in our __pte_free_tlb() code path,
so that the PG_table flag is cleared before we free the pte page.

Note that I had to change the type of pte_free() to be static from
extern. Otherwise you get a lot of warnings like this:

./arch/m68k/include/asm/mcf_pgalloc.h:80:2: warning: ‘pgtable_page_dtor’ is static but used in inline function ‘pte_free’ which is not static
  pgtable_page_dtor(page);
  ^

And making it static is consistent with our use of this in the other
m68k pgalloc definitions of pte_free().
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
CC: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/include/asm/mcf_pgalloc.h

@@ -44,6 +44,7 @@ extern inline pmd_t *pmd_alloc_kernel(pgd_t *pgd, unsigned long address)
 static inline void __pte_free_tlb(struct mmu_gather *tlb, pgtable_t page,
 				  unsigned long address)
 {
+	pgtable_page_dtor(page);
 	__free_page(page);
 }
 
@@ -74,8 +75,9 @@ static inline struct page *pte_alloc_one(struct mm_struct *mm,
 	return page;
 }
 
-extern inline void pte_free(struct mm_struct *mm, struct page *page)
+static inline void pte_free(struct mm_struct *mm, struct page *page)
 {
+	pgtable_page_dtor(page);
 	__free_page(page);
 }