mm: page_alloc: make compound_page_dtors static

It's only used inside page_alloc.c now. So make it static and remove the
declaration in mm.h.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Miaohe Lin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/mm.h

···
 #endif
 	NR_COMPOUND_DTORS,
 };
-extern compound_page_dtor * const compound_page_dtors[NR_COMPOUND_DTORS];
 
 static inline void folio_set_compound_dtor(struct folio *folio,
 		enum compound_dtor_id compound_dtor)

mm/page_alloc.c

···
 #endif
 };
 
-compound_page_dtor * const compound_page_dtors[NR_COMPOUND_DTORS] = {
+static compound_page_dtor * const compound_page_dtors[NR_COMPOUND_DTORS] = {
 	[NULL_COMPOUND_DTOR] = NULL,
 	[COMPOUND_PAGE_DTOR] = free_compound_page,
 #ifdef CONFIG_HUGETLB_PAGE