tracing: probeevent: Fix uninitialized used of offset in parse args

Dan's smatch utility found an uninitialized use of offset in a path in
parse_probe_args(). Unless an offset is specifically specified for commands
that allow them, it should default to zero.

Link: http://lkml.kernel.org/r/20181012134246.5doqaobxunlqqs53@mwanda


Fixes: 53305928 ("tracing: probeevent: Introduce new argument fetching code")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com> (smatch)
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>