fgraph: Use str_plural() in test_graph_storage_single()

fgraph: Use str_plural() in test_graph_storage_single()

Use existing str_plural() function rather than duplicating its
implementation.

./kernel/trace/trace_selftest.c:880:56-60: opportunity for str_plural(size).

Link: https://lore.kernel.org/linux-trace-kernel/20240618072014.20855-1-jiapeng.chong@linux.alibaba.com
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=9349
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>