LU-6529 ldlm: improve proc interface of lock reclaim

LU-6529 ldlm: improve proc interface of lock reclaim

Rename variables of ldlm_watermark_low & ldlm_watermark_high to
ldlm_reclaim_threshold & ldlm_lock_limit, and introduce two
additional variables of ldlm_reclaim_threshold_mb and
ldlm_lock_limit_mb for proc interface, so that we needn't to
convert the MB to locknr each time;

Adjust the proc name from watermark_low_mb & watermark_high_mb to
lock_reclaim_threshold_mb & lock_limit_mb;

Remove the misleading comment at the top of ldlm_reclaim.c, add more
comments to functions in ldlm_reclaim.c;

Export the total granted locks via proc for debug purpose;

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I161e088513ba2319cfdfe0d703512b4d9eeeebf1
Reviewed-on: http://review.whamcloud.com/16123
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>