riscv: Fix sparse warning in __get_user_error()

commit fef7ded upstream.

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 <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508062321.gHv4kvuY-lkp@intel.com/
Fixes: f6bff78 ("riscv: uaccess: use 'asm_goto_output' for get_user()")
Cc: stable@vger.kernel.org
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Cyril Bur <cyrilbur@tenstorrent.com>
Link: https://lore.kernel.org/r/20250903-dev-alex-sparse_warnings_v1-v1-1-7e6350beb700@rivosinc.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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)