LU-1453 scrub: rename confused name full_scrub_speed

LU-1453 scrub: rename confused name full_scrub_speed

Such name means if the rate of auto detected inconsistent
OI mappings exceeds the specified threshold, then trigger
OI scrub to scan the whole device. To avoid confusion,
rename it as full_scrub_threshold_rate.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I331fcd60c01828f5b689f4d1dc4601f38c3e9cf4
Reviewed-on: http://review.whamcloud.com/12958
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>