staging/lustre/libcfs: protect kkuc_groups from write access

Since reg->kr_fp can be changed inside the foreach loop,
kkuc_groups must be write protected, and not just read protected.

This should fix the following oops, which could happen if two different
threads simultaneously execute the function, and EPIPE is returned.

PID: 24385  TASK: ffff88012da5f500  CPU: 1   COMMAND: "ldlm_cb00_056"
 #0 [ffff88012db55810] machine_kexec at ffffffff81038f3b
 #1 [ffff88012db55870] crash_kexec at ffffffff810c59f2
 #2 [ffff88012db55940] oops_end at ffffffff8152b7f0
 #3 [ffff88012db55970] no_context at ffffffff8104a00b
 #4 [ffff88012db559c0] __bad_area_nosemaphore at ffffffff8104a295
 #5 [ffff88012db55a10] bad_area_nosemaphore at ffffffff8104a363
 #6 [ffff88012db55a20] __do_page_fault at ffffffff8104aabf
 #7 [ffff88012db55b40] do_page_fault at ffffffff8152d73e
 #8 [ffff88012db55b70] page_fault at ffffffff8152aaf5
    [exception RIP: fput+9]
    RIP: ffffffff8118a509  RSP: ffff88012db55c20  RFLAGS: 00010246
    RAX: 00000000ffffffe0  RBX: ffff8800a8ea4fc0  RCX: 0000000000000000
    RDX: ffffffffa03c9eb0  RSI: 0000000000000000  RDI: 0000000000000000
    RBP: ffff88012db55c20   R8: 00000000ffffff0a   R9: 00000000fffffffc
    R10: 0000000000000001  R11: 282064656c696166  R12: ffffffffa03c9c60
    R13: ffff88005df240f8  R14: 0000000000000000  R15: ffff88013b4ca000
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #9 [ffff88012db55c28] libcfs_kkuc_group_put at ffffffffa0388044 [libcfs]
[ptlrpc]

Signed-off-by: frank zago <fzago@cray.com>
Reviewed-on: http://review.whamcloud.com/11355
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5458
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>