powerpc/uaccess: Allow get_user() with bitwise types

At the moment, if p and x are both of the same bitwise type
(eg. __le32), get_user(x, p) produces a sparse warning.

This is because *p is loaded into a long then cast back to typeof(*p).

When typeof(*p) is a bitwise type (which is uncommon), such a cast needs
__force, otherwise sparse produces a warning.

For non-bitwise types __force should have no effect, and should not hide
any legitimate errors.

Note that we are casting to typeof(*p) not typeof(x). Even with the
cast, if x and *p are of different types we should get the warning, so I
think we are not loosing the ability to detect any actual errors.

virtio would like to use bitwise types with get_user() so fix these
spurious warnings by adding __force.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
[mpe: Fill in changelog with more details]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/uaccess.h

@@ -284,7 +284,7 @@ do {								\
 	if (!is_kernel_addr((unsigned long)__gu_addr))		\
 		might_fault();					\
 	__get_user_size(__gu_val, __gu_addr, (size), __gu_err);	\
-	(x) = (__typeof__(*(ptr)))__gu_val;			\
+	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
 	__gu_err;						\
 })
 #endif /* __powerpc64__ */
@@ -297,7 +297,7 @@ do {								\
 	might_fault();							\
 	if (access_ok(VERIFY_READ, __gu_addr, (size)))			\
 		__get_user_size(__gu_val, __gu_addr, (size), __gu_err);	\
-	(x) = (__typeof__(*(ptr)))__gu_val;				\
+	(x) = (__force __typeof__(*(ptr)))__gu_val;				\
 	__gu_err;							\
 })
 
@@ -308,7 +308,7 @@ do {								\
 	const __typeof__(*(ptr)) __user *__gu_addr = (ptr);	\
 	__chk_user_ptr(ptr);					\
 	__get_user_size(__gu_val, __gu_addr, (size), __gu_err);	\
-	(x) = (__typeof__(*(ptr)))__gu_val;			\
+	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
 	__gu_err;						\
 })