fprobe: Check rethook_alloc() return in rethook initialization

Check if fp->rethook succeeded to be allocated. Otherwise, if
rethook_alloc() fails, then we end up dereferencing a NULL pointer in
rethook_add_node().

Link: https://lore.kernel.org/all/20221025031209.954836-1-rafaelmendsr@gmail.com/



Fixes: 5b0ab789 ("fprobe: Add exit_handler support")
Cc: stable@vger.kernel.org
Signed-off-by: Rafael Mendonca <rafaelmendsr@gmail.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>