ftrace: Rename ftrace_regs_return_value to ftrace_regs_get_return_value

ftrace: Rename ftrace_regs_return_value to ftrace_regs_get_return_value

Rename ftrace_regs_return_value to ftrace_regs_get_return_value as same as
other ftrace_regs_get/set_* APIs. arm64 and riscv are already using this
new name.

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>
Link: https://lore.kernel.org/172895573350.107311.7564634260652361511.stgit@devnote2
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>