The list_lock can be taken in hardirq context when do_drain() is being
called via IPI on all cores, and therefore lockdep complains about it,
because it can't be preempted on PREEMPT_RT.

That's not a real issue, as SLAB can't be built on PREEMPT_RT anyway, but
we still want to get rid of the warning on non-PREEMPT_RT builds.

Annotate it therefore as a raw lock in order to get rid of he lockdep
warning below.

	 =============================
	 [ BUG: Invalid wait context ]
	 6.1.0-rc1-00134-ge35184f3 #4 Not tainted
	 -----------------------------
	 swapper/3/0 is trying to lock:
	 ffff8bc88086dc18 (&parent->list_lock){..-.}-{3:3}, at: do_drain+0x57/0xb0
	 other info that might help us debug this:
	 context-{2:2}
	 no locks held by swapper/3/0.
	 stack backtrace:
	 CPU: 3 PID: 0 Comm: swapper/3 Not tainted 6.1.0-rc1-00134-ge35184f3 #4
	 Hardware name: LENOVO 20K5S22R00/20K5S22R00, BIOS R0IET38W (1.16 ) 05/31/2017
	 Call Trace:
	  <IRQ>
	  dump_stack_lvl+0x6b/0x9d
	  __lock_acquire+0x1519/0x1730
	  ? build_sched_domains+0x4bd/0x1590
	  ? __lock_acquire+0xad2/0x1730
	  lock_acquire+0x294/0x340
	  ? do_drain+0x57/0xb0
	  ? sched_clock_tick+0x41/0x60
	  _raw_spin_lock+0x2c/0x40
	  ? do_drain+0x57/0xb0
	  do_drain+0x57/0xb0
	  __flush_smp_call_function_queue+0x138/0x220
	  __sysvec_call_function+0x4f/0x210
	  sysvec_call_function+0x4b/0x90
	  </IRQ>
	  <TASK>
	  asm_sysvec_call_function+0x16/0x20
	 RIP: 0010:mwait_idle+0x5e/0x80
	 Code: 31 d2 65 48 8b 04 25 80 ed 01 00 48 89 d1 0f 01 c8 48 8b 00 a8 08 75 14 66 90 0f 00 2d 0b 78 46 00 31 c0 48 89 c1 fb 0f 01 c9 <eb> 06 fb 0f 1f 44 00 00 65 48 8b 04 25 80 ed 01 00 f0 80 60 02 df
	 RSP: 0000:ffffa90940217ee0 EFLAGS: 00000246
	 RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
	 RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff9bb9f93a
	 RBP: 0000000000000003 R08: 0000000000000001 R09: 0000000000000001
	 R10: ffffa90940217ea8 R11: 0000000000000000 R12: ffffffffffffffff
	 R13: 0000000000000000 R14: ffff8bc88127c500 R15: 0000000000000000
	  ? default_idle_call+0x1a/0xa0
	  default_idle_call+0x4b/0xa0
	  do_idle+0x1f1/0x2c0
	  ? _raw_spin_unlock_irqrestore+0x56/0x70
	  cpu_startup_entry+0x19/0x20
	  start_secondary+0x122/0x150
	  secondary_startup_64_no_verify+0xce/0xdb
	  </TASK>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>