riscv: stacktrace: Fixup ftrace_graph_ret_addr retp argument

The 'retp' is a pointer to the return address on the stack, so we
must pass the current return address pointer as the 'retp'
argument to ftrace_push_return_trace(). Not parent function's
return address on the stack.

Fixes: b785ec12 ("riscv/ftrace: Add HAVE_FUNCTION_GRAPH_RET_ADDR_PTR support")
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Alan Kao <alankao@andestech.com>
Cc: Palmer Dabbelt <palmer@rivosinc.com>