libtracefs: Implement tracefs_kretprobe_raw()

Add an interface that is similar to tracefs_kprobe_raw() but creates a
"kretprobe". The difference between a kprobe and a kretprobe is that a
kretprobe comes at the end of a function.

See Documentation/trace/kprobetrace.rst in the Linux kernel source code
for more information. Note, kprobes are started with "p" and kretprobe
lines start with an "r".

Link: https://lore.kernel.org/linux-trace-devel/20210702201727.169080-4-rostedt@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>