riscv: Fix ftrace_graph_ret_addr() to use the correct task pointer

riscv: Fix ftrace_graph_ret_addr() to use the correct task pointer

The walk_stackframe() function is used to unwind the stack of a given
task. When function graph tracing is enabled, ftrace_graph_ret_addr()
is called to resolve the original return address if it was modified by
the tracer.

The current code incorrectly passes 'current' instead of 'task' to
ftrace_graph_ret_addr(). This causes incorrect return address resolution
when unwinding a stack of a different task (e.g., when the task is
blocked in __switch_to).

Fix this by passing 'task' instead of 'current' to match the behavior
of other architectures (arm64, loongarch, powerpc, s390, x86).

Signed-off-by: Rui Qi <qirui.001@bytedance.com>
Link: https://patch.msgid.link/20260408092915.46408-1-qirui.001@bytedance.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>