tracing/histograms: Simplify last_cmd_set()

Turn a kzalloc()+strcpy()+strncat() into an equivalent and less verbose
kasprintf().

Link: https://lore.kernel.org/linux-trace-kernel/30b6fb04dadc10a03cc1ad08f5d8a93ef623a167.1697899346.git.christophe.jaillet@wanadoo.fr



Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Mukesh ojha <quic_mojha@quicinc.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>