fprobe: Fix smatch type mismatch warning

Fix the type mismatching warning of 'rethook_node vs fprobe_rethook_node'
found by Smatch.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/164802092611.1732982.12268174743437084619.stgit@devnote2