LU-11157 obd: keep dirty_max_pages a round number of MB

LU-11157 obd: keep dirty_max_pages a round number of MB

In client_adjust_max_dirty() ensure that the dirty pages limit is
always divisible by 256 so that it may faithfully be represented in MB
as is the case when the max_dirty_mb parameters are used.

Test-Parameters: trivial

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I8e2fbdd4bf253a46e2951e7840484ab6a617fbe2
Reviewed-on: https://review.whamcloud.com/32831
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>