LU-3701 ldlm: refine LU-2665 patch for POSIX compliance

Follow-on to patch introduced to fix LU-2665 ticket (Gerrit
Change at http://review.whamcloud.com/6415 with Change-Id:
I8faa331712abeadee46eabe111ee1c23a05840d5).
Original patch introduced regressions against POSIX test suite
(fcntl.18/fcntl.35 tests in LSB-VSX POSIX test suite at
http://www.opengroup.org/testing/linux-test/lsb-vsx.html),
so the idea is to only resend F_UNLCKs to have both LU-2665 bug
and POSIX test suite happy.

Test-Parameters: testlist=posix
Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I8d325cb7039bb3f3f74f2a9829cbaea760d9e20f
Reviewed-on: http://review.whamcloud.com/7453
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>