riscv; fix __user annotation in save_v_state()

riscv; fix __user annotation in save_v_state()

The save_v_state() is technically sending a __user pointer through
__put_user() and thus is generating a sparse warning so force the
value to be "void *" to fix:

arch/riscv/kernel/signal.c:94:16: warning: incorrect type in initializer (different address spaces)
arch/riscv/kernel/signal.c:94:16: expected void *__val
arch/riscv/kernel/signal.c:94:16: got void [noderef] __user *[assigned] datap

Fixes: 8ee0b41898fa26f66e32 ("riscv: signal: Add sigcontext save/restore for vector")
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://lore.kernel.org/r/20231123142708.261733-1-ben.dooks@codethink.co.uk
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>