LU-6775 ldlm: reduce mem footprint of ldlm_resource

LU-6775 ldlm: reduce mem footprint of ldlm_resource

- Removing the unused lr_most_restr, reduced 4 bytes;
- Allocating lr_itree only for LDLM_EXTENT resource, reduced
  120 bytes;
- Moving fields around to eliminate holes, eliminated 3 holes,
  reduced 4 bytes;
- Making lr_contention_time and lr_lvb_inode into union,
  reduced 8 bytes;

Reduced 136 bytes in total.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I232a2669d9f5eaee35eb9ff8313d86d4ac297bc1
Reviewed-on: http://review.whamcloud.com/15485
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>