LU-7218 osc: fix max_dirty_mb tunable setting limit

LU-7218 osc: fix max_dirty_mb tunable setting limit

The OSC tunable max_dirty_mb must be set to a value strictly lower
than 2048, as it is assumed by OSS in ofd_grant_alloc() routine.

Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Change-Id: I66a0927c69749cdbb9cd48459af67a57c3e25af0
Reviewed-on: http://review.whamcloud.com/16652
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>