selftests/riscv: ptrace: Fix memory leak of regset_data in vector tests

selftests/riscv: ptrace: Fix memory leak of regset_data in vector tests

The regset_data buffer allocated with calloc() in the parent process
of several vector ptrace tests is never freed before returning,
causing memory leaks in:

- ptrace_v_not_enabled
- ptrace_v_early_debug
- ptrace_v_syscall_clobbering
- v_csr_invalid/ptrace_v_invalid_values
- v_csr_valid/ptrace_v_valid_values

Add free(regset_data) before kill(pid, SIGKILL) to release the
allocated buffer.

Signed-off-by: Wang Yan <wangyan01@kylinos.cn>
Reviewed-by: Sergey Matyukevich <geomatsi@gmail.com>
Link: https://patch.msgid.link/20260710083437.489648-1-wangyan01@kylinos.cn
[pjw@kernel.org: Fixed Sergey's E-mail address]
Signed-off-by: Paul Walmsley <pjw@kernel.org>