LU-7795 quota: tuneable soft least qunit

LU-7795 quota: tuneable soft least qunit

Introduce a tuneable least qunit for soft block quota, which is
used when soft limit is exceeded to retain an acceptable write
performance.

Remove the stale declarations of quota proc function.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I172d1d08494418a6f7bc261f022c587da34b784d
Reviewed-on: http://review.whamcloud.com/18896
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>