tracing: fprobe events: Fix possible UAF on modules

Commit ac91052f ("tracing: tprobe-events: Fix leakage of module
refcount") moved try_module_get() from __find_tracepoint_module_cb()
to find_tracepoint() caller, but that introduced a possible UAF
because the module can be unloaded before try_module_get(). In this
case, the module object should be freed too. Thus, try_module_get()
does not only fail but may access to the freed object.

To avoid that, try_module_get() in __find_tracepoint_module_cb()
again.

Link: https://lore.kernel.org/all/174342990779.781946.9138388479067729366.stgit@devnote2/



Fixes: ac91052f ("tracing: tprobe-events: Fix leakage of module refcount")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>