sh: cmpxchg: fix a bit shift bug in big_endian os

Correct bitoff in big endian OS.
Current code works correctly for 1 byte but not for 2 bytes.

Fixes: 3226aad81a ("sh: support 1 and 2 byte xchg")
Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rich Felker <dalias@libc.org>

arch/sh/include/asm/cmpxchg-xchg.h

@@ -21,7 +21,7 @@ static inline u32 __xchg_cmpxchg(volatile void *ptr, u32 x, int size)
 	int off = (unsigned long)ptr % sizeof(u32);
 	volatile u32 *p = ptr - off;
 #ifdef __BIG_ENDIAN
-	int bitoff = (sizeof(u32) - 1 - off) * BITS_PER_BYTE;
+	int bitoff = (sizeof(u32) - size - off) * BITS_PER_BYTE;
 #else
 	int bitoff = off * BITS_PER_BYTE;
 #endif