kprobes: Fix compiler warning for !CONFIG_KPROBES_ON_FTRACE

commit 10de795a5addd1962406796a6e13ba6cc0fc6bee upstream.

Fix compiler warning(as show below) for !CONFIG_KPROBES_ON_FTRACE.

kernel/kprobes.c: In function 'kill_kprobe':
kernel/kprobes.c:1116:33: warning: statement with no effect
[-Wunused-value]
 1116 | #define disarm_kprobe_ftrace(p) (-ENODEV)
      |                                 ^
kernel/kprobes.c:2154:3: note: in expansion of macro
'disarm_kprobe_ftrace'
 2154 |   disarm_kprobe_ftrace(p);

Link: https://lore.kernel.org/r/20200805142136.0331f7ea@canb.auug.org.au
Link: https://lkml.kernel.org/r/20200805172046.19066-1-songmuchun@bytedance.com



Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 0cb2f1372baa ("kprobes: Fix NULL pointer dereference at kprobe_ftrace_handler")
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>