powerpc/mm: Fix build warnings with CONFIG_TRANSPARENT_HUGEPAGE disabled

Building with CONFIG_TRANSPARENT_HUGEPAGE disabled causes the following
build wearnings;

powerpc/arch/powerpc/include/asm/mmu-hash64.h: In function ‘__hash_page_thp’:
powerpc/arch/powerpc/include/asm/mmu-hash64.h:354: warning: no return statement in function returning non-void

This patch adds a return -1 to the static inline for __hash_page_thp()
to correct the warnings.
Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/mmu-hash64.h

@@ -351,6 +351,7 @@ static inline int __hash_page_thp(unsigned long ea, unsigned long access,
 				  int ssize, unsigned int psize)
 {
 	BUG();
+	return -1;
 }
 #endif
 extern void hash_failure_debug(unsigned long ea, unsigned long access,