kprobes: fix a memory leak in function pre_handler_kretprobe()

kprobes: fix a memory leak in function pre_handler_kretprobe()

In function pre_handler_kretprobe(), the allocated kretprobe_instance
object will get leaked if the entry_handler callback returns non-zero.
This may cause all the preallocated kretprobe_instance objects exhausted.

This issue can be reproduced by changing
samples/kprobes/kretprobe_example.c to probe "mutex_unlock".  And the fix
is straightforward: just put the allocated kretprobe_instance object back
onto the free_instances list.

[[email protected]: use raw_spin_lock/unlock]
Signed-off-by: Jiang Liu <[email protected]>
Acked-by: Jim Keniston <[email protected]>
Acked-by: Ananth N Mavinakayanahalli <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Anil S Keshavamurthy <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>