arm64: stacktrace: fix the usage of ftrace_graph_ret_addr()

ftrace_graph_ret_addr() takes an 'idx' integer pointer that is used to
optimize the stack unwinding process. arm64 currently passes `NULL` for
this parameter which stops it from utilizing these optimizations.

Further, the current code for ftrace_graph_ret_addr() will just return
the passed in return address if it is NULL which will break this usage.

Pass a valid integer pointer to ftrace_graph_ret_addr() similar to
x86_64's stack unwinder.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Fixes: 29c1c24a ("function_graph: Fix up ftrace_graph_ret_addr()")
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20240618162342.28275-1-puranjay@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>