kprobes: Fix build errors with CONFIG_KRETPROBES=n

kprobes: Fix build errors with CONFIG_KRETPROBES=n

Max Filippov reported:

When building kernel with CONFIG_KRETPROBES=n kernel/kprobes.c
compilation fails with the following messages:

  kernel/kprobes.c: In function ‘recycle_rp_inst’:
  kernel/kprobes.c:1273:32: error: implicit declaration of function
                                   ‘get_kretprobe’

  kernel/kprobes.c: In function ‘kprobe_flush_task’:
  kernel/kprobes.c:1299:35: error: ‘struct task_struct’ has no member
                                   named ‘kretprobe_instances’

This came from the commit d741bf41d7c7 ("kprobes: Remove
kretprobe hash") which introduced get_kretprobe() and
kretprobe_instances member in task_struct when CONFIG_KRETPROBES=y,
but did not make recycle_rp_inst() and kprobe_flush_task()
depending on CONFIG_KRETPORBES.

Since those functions are only used for kretprobe, move those
functions into #ifdef CONFIG_KRETPROBE area.

Link: https://lkml.kernel.org/r/165163539094.74407.3838114721073251225.stgit@devnote2
Reported-by: Max Filippov <[email protected]>
Fixes: d741bf41d7c7 ("kprobes: Remove kretprobe hash")
Cc: "Naveen N . Rao" <[email protected]>
Cc: Anil S Keshavamurthy <[email protected]>
Cc: "David S . Miller" <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Signed-off-by: Masami Hiramatsu <[email protected]>
Tested-by: Max Filippov <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>