riscv: Fix sparse warning in __get_user_error()

We used to assign 0 to x without an appropriate cast which results in
sparse complaining when x is a pointer:

>> block/ioctl.c:72:39: sparse: sparse: Using plain integer as NULL pointer

So fix this by casting 0 to the correct type of x.

Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Fixes: f6bff78 ("riscv: uaccess: use 'asm_goto_output' for get_user()")
Cc: [email protected]
Signed-off-by: Alexandre Ghiti <[email protected]>
Reviewed-by: Clément Léger <[email protected]>
Reviewed-by: Cyril Bur <[email protected]>
Link: https://lore.kernel.org/r/20250903-dev-alex-sparse_warnings_v1-v1-1-7e6350beb700@rivosinc.com
Signed-off-by: Paul Walmsley <[email protected]>

Author: Alexandre Ghiti

arch/riscv/include/asm/uaccess.h

@@ -209,7 +209,7 @@ do {									\
 		err = 0;						\
 		break;							\
 __gu_failed:								\
-		x = 0;							\
+		x = (__typeof__(x))0;					\
 		err = -EFAULT;						\
 } while (0)