kprobes/x86: Release insn_slot in failure path

The following commit:

  003002e0 ("kprobes: Fix arch_prepare_kprobe to handle copy insn failures")

returns an error if the copying of the instruction, but does not release
the allocated insn_slot.

Clean up correctly.
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: David S . Miller <davem@davemloft.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 003002e0 ("kprobes: Fix arch_prepare_kprobe to handle copy insn failures")
Link: http://lkml.kernel.org/r/150064834183.6172.11694375818447664416.stgit@devboxSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/kprobes/core.c

@@ -457,6 +457,8 @@ static int arch_copy_kprobe(struct kprobe *p)
 
 int arch_prepare_kprobe(struct kprobe *p)
 {
+	int ret;
+
 	if (alternatives_text_reserved(p->addr, p->addr))
 		return -EINVAL;
 
@@ -467,7 +469,13 @@ int arch_prepare_kprobe(struct kprobe *p)
 	if (!p->ainsn.insn)
 		return -ENOMEM;
 
-	return arch_copy_kprobe(p);
+	ret = arch_copy_kprobe(p);
+	if (ret) {
+		free_insn_slot(p->ainsn.insn, 0);
+		p->ainsn.insn = NULL;
+	}
+
+	return ret;
 }
 
 void arch_arm_kprobe(struct kprobe *p)