riscv: Fix sparse warning in __get_user_error()

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 <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508062321.gHv4kvuY-lkp@intel.com/
Fixes: f6bff7827a48 ("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>