LU-12828 ldlm: FLOCK request can be processed twice

LU-12828 ldlm: FLOCK request can be processed twice

Original request can be processed after resend
request, so it can create a lock on MDT without
client lock or unlock other lock.

Make flock enqueue to use modify RPC slot.

Change-Id: Icfee202fe2e389beda1116f78f8b933c7ea182fb
Cray-bug-id: LUS-5739
Signed-off-by: Andriy Skulysh <c17819@cray.com>
Signed-off-by: Vitaly Fertman <c17818@cray.com>
Reviewed-by: Alexander Boyko <c17825@cray.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-on: https://review.whamcloud.com/36340
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>