[PATCH] fix race in idr code

I ran into a bug where the kernel died in the idr code:

cpu 0x1d: Vector: 300 (Data Access) at [c000000b7096f710]
    pc: c0000000001f8984: .idr_get_new_above_int+0x140/0x330
    lr: c0000000001f89b4: .idr_get_new_above_int+0x170/0x330
    sp: c000000b7096f990
   msr: 800000000000b032
   dar: 0
 dsisr: 40010000
  current = 0xc000000b70d43830
  paca    = 0xc000000000556900
    pid   = 2022, comm = hwup
1d:mon> t
[c000000b7096f990] c0000000000d2ad8 .expand_files+0x2e8/0x364 (unreliable)
[c000000b7096faa0] c0000000001f8bf8 .idr_get_new_above+0x18/0x68
[c000000b7096fb20] c00000000002a054 .init_new_context+0x5c/0xf0
[c000000b7096fbc0] c000000000049dc8 .copy_process+0x91c/0x1404
[c000000b7096fcd0] c00000000004a988 .do_fork+0xd8/0x224
[c000000b7096fdc0] c00000000000ebdc .sys_clone+0x5c/0x74
[c000000b7096fe30] c000000000008950 .ppc_clone+0x8/0xc