ftrace: Use arch_ftrace_regs() for ftrace_regs_*() macros

ftrace: Use arch_ftrace_regs() for ftrace_regs_*() macros

Since the arch_ftrace_get_regs(fregs) is only valid when the
FL_SAVE_REGS is set, we need to use `&arch_ftrace_regs()->regs` for
ftrace_regs_*() APIs because those APIs are for ftrace_regs, not
complete pt_regs.

Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Florent Revest <revest@chromium.org>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: bpf <bpf@vger.kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Alan Maguire <alan.maguire@oracle.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/172895572290.107311.16057631001860177198.stgit@devnote2
Fixes: e4cf33ca4812 ("ftrace: Consolidate ftrace_regs accessor functions for archs using pt_regs")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>