LU-4974 lod: use pool_key() to access key

LU-4974 lod: use pool_key() to access key

In pool_hashkey_keycmp() use the pool_key() function instead of open
coding the key access.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Ia618e25ed4f086321865a9439142e43405500c1e
Reviewed-on: http://review.whamcloud.com/10244
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>