staging/lustre/ldlm: engage ELC for all ldlm enqueue req

If there is no request passed into ldlm_cli_enqueue, the enqueue
request will not engage ELC to drop unneeded locks. currently,
this kind of request is mainly related to EXTENT locks enqueue
requests (except for glimpse EXTENT lock for it has an intent).

Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-on: http://review.whamcloud.com/21739
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8209


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Vitaly Fertman <vitaly.fertman@seagate.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>