tracing: have TRACE_EVENT macro use __flags to not shadow parameter

The generated functions of TRACE_EVENT uses "flags" in one of the
sub macros which shadows a parameter in the outside macro.

Simple fix is to make the submacro use __flags instead.

Discovered by sparse.

Reported-by: Jaswinder Singh Rajput <jaswinder@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>