tracing: Fix wrong return in kprobe_event_gen_test.c

tracing: Fix wrong return in kprobe_event_gen_test.c

Overwriting the error code with the deletion result may cause the
function to return 0 despite encountering an error. Commit b111545d26c0
("tracing: Remove the useless value assignment in
test_create_synth_event()") solves a similar issue by
returning the original error code, so this patch does the same.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Link: https://lore.kernel.org/linux-trace-kernel/20230131075818.5322-1-aagusev@ispras.ru
Signed-off-by: Anton Gusev <aagusev@ispras.ru>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>