powerpc/kasan: Declare kasan_init_region() weak

In order to alloc sub-arches to alloc KASAN regions using optimised
methods (Huge pages on 8xx, BATs on BOOK3S, ...), declare
kasan_init_region() weak.

Also make kasan_init_shadow_page_tables() accessible from outside,
so that it can be called from the specific kasan_init_region()
functions if needed.

And populate remaining KASAN address space only once performed
the region mapping, to allow 8xx to allocate hugepd instead of
standard page tables for mapping via 8M hugepages.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3c1ce419fa1b5a4171b92d7fb16455ca17e1b96d.1589866984.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/kasan.h

@@ -34,5 +34,8 @@ static inline void kasan_init(void) { }
 static inline void kasan_late_init(void) { }
 #endif
 
+int kasan_init_shadow_page_tables(unsigned long k_start, unsigned long k_end);
+int kasan_init_region(void *start, size_t size);
+
 #endif /* __ASSEMBLY */
 #endif