tracing: eprobe-event: Allocate string buffers from heap

Allocate temporary string buffers for parsing eprobe-events
from heap instead of stack.

Link: https://lore.kernel.org/all/175323428599.57270.988038042425748956.stgit@devnote2/



Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>