tracing: Enforce passing in filter=NULL to create_filter()

There's some inconsistency with what to set the output parameter filterp
when passing to create_filter(..., struct event_filter **filterp).

Whatever filterp points to, should be NULL when calling this function. The
create_filter() calls create_filter_start() with a pointer to a local
"filter" variable that is set to NULL. The create_filter_start() has a
WARN_ON() if the passed in pointer isn't pointing to a value set to NULL.

Ideally, create_filter() should pass the filterp variable it received to
create_filter_start() and not hide it as with a local variable, this allowed
create_filter() to fail, and not update the passed in filter, and the caller
of create_filter() then tried to free filter, which was never initialized to
anything, causing memory corruption.

Link: http://lkml.kernel.org/r/00000000000032a0c30569916870@google.com



Fixes: 80765597 ("tracing: Rewrite filter logic to be simpler and faster")
Reported-by:  <syzbot+dadcc936587643d7f568@syzkaller.appspotmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>