mm: allow can_split_folio() to be called when THP are disabled

The call to can_split_folio() in vmscan is currently guarded by a test of
PageTransHuge() so the BUILD_BUG() is eliminated if THP are disabled.  The
next patch replaces that test with folio_test_large() which may be true,
even when THP are disabled.  However, if THP are disabled, we cannot
split, so an unconditional return of false is appropriate.

Link: https://lkml.kernel.org/r/20220504182857.4013401-15-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/huge_mm.h

@@ -348,7 +348,6 @@ static inline void prep_transhuge_page(struct page *page) {}
 static inline bool
 can_split_folio(struct folio *folio, int *pextra_pins)
 {
-	BUILD_BUG();
 	return false;
 }
 static inline int