riscv: mmap(): use unsigned offset type in riscv_sys_mmap

riscv: mmap(): use unsigned offset type in riscv_sys_mmap

The variable type of offset should be consistent with the relevant
interfaces of mmap which described in commit 295f10061af0 ("syscalls:
mmap(): use unsigned offset type consistently"). Otherwise, a user input
with the top bit set would result in a negative page offset rather than a
large one.

Signed-off-by: Jessica Liu <liu.xuemei1@zte.com.cn>
Tested-by: Han Gao <rabenda.cn@gmail.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Nutty Liu <liujingqi@lanxincomputing.com>
Link: https://lore.kernel.org/r/20250801104948133AaMr5S6E382PbNNhoJgHA@zte.com.cn
[pjw@kernel.org: hand-applied mangled patch; fixed checkpatch error]
Signed-off-by: Paul Walmsley <pjw@kernel.org>