LU-2816 llite: Set RAS_INCREASE_STEP correctly

RAS_INCREASE_STEP is by pages instead of by bytes.

However, I found it caused performance loss by setting it to be 4MB so
I set it back to 1MB. After 4MB RPC is enabled by default, more work
should be done to pick up a right value of max_read_ahead_mb to
maximize performance gain of 4MB RPC.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ic000f266cfefc827e112e03f76cf467c73ba88ad
Reviewed-on: http://review.whamcloud.com/5691
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>