powerpc/32s: Fix shift-out-of-bounds in KASAN init

================================================================================
UBSAN: shift-out-of-bounds in arch/powerpc/mm/kasan/book3s_32.c:22:23
shift exponent -1 is negative
CPU: 0 PID: 0 Comm: swapper Not tainted 5.15.5-gentoo-PowerMacG4 #9
Call Trace:
[c214be60] [c0ba0048] dump_stack_lvl+0x80/0xb0 (unreliable)
[c214be80] [c0b99288] ubsan_epilogue+0x10/0x5c
[c214be90] [c0b98fe0] __ubsan_handle_shift_out_of_bounds+0x94/0x138
[c214bf00] [c1c0f010] kasan_init_region+0xd8/0x26c
[c214bf30] [c1c0ed84] kasan_init+0xc0/0x198
[c214bf70] [c1c08024] setup_arch+0x18/0x54c
[c214bfc0] [c1c037f0] start_kernel+0x90/0x33c
[c214bff0] [00003610] 0x3610
================================================================================

This happens when the directly mapped memory is a power of 2.

Fix it by checking the shift and set the result to 0 when shift is -1

Fixes: 7974c47 ("powerpc/32s: Implement dedicated kasan_init_region()")
Reported-by: Erhard Furtner <erhard_f@mailbox.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=215169
Link: https://lore.kernel.org/r/15cbc3439d4ad988b225e2119ec99502a5cc6ad3.1638261744.git.christophe.leroy@csgroup.eu

arch/powerpc/mm/kasan/book3s_32.c

@@ -19,7 +19,8 @@ int __init kasan_init_region(void *start, size_t size)
 	block = memblock_alloc(k_size, k_size_base);
 
 	if (block && k_size_base >= SZ_128K && k_start == ALIGN(k_start, k_size_base)) {
-		int k_size_more = 1 << (ffs(k_size - k_size_base) - 1);
+		int shift = ffs(k_size - k_size_base);
+		int k_size_more = shift ? 1 << (shift - 1) : 0;
 
 		setbat(-1, k_start, __pa(block), k_size_base, PAGE_KERNEL);
 		if (k_size_more >= SZ_128K)