powerpc/kprobes: Remove preempt disable around call to get_kprobe() in arch_prepare_kprobe()

arch_prepare_kprobe() is called from register_kprobe() via
prepare_kprobe(), or through register_aggr_kprobe(), both with the
kprobe_mutex held. Per the comment for get_kprobe():
  /*
   * This routine is called either:
   *	- under the 'kprobe_mutex' - during kprobe_[un]register().
   *				OR
   *	- with preemption disabled - from architecture specific code.
   */

As such, there is no need to disable preemption around the call to
get_kprobe(). Drop the same.
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1043d06a0affed83a4a46dd29466e72820ee215d.1666262278.git.naveen.n.rao@linux.vnet.ibm.com

arch/powerpc/kernel/kprobes.c

@@ -158,9 +158,7 @@ int arch_prepare_kprobe(struct kprobe *p)
 		printk("Cannot register a kprobe on the second word of prefixed instruction\n");
 		ret = -EINVAL;
 	}
-	preempt_disable();
 	prev = get_kprobe(p->addr - 1);
-	preempt_enable_no_resched();
 
 	/*
 	 * When prev is a ftrace-based kprobe, we don't have an insn, and it