ocfs2: make 'buffered' as the default coherency option

ocfs2: make 'buffered' as the default coherency option

Orabug: 17988729

Customers upgrading to uek2 and above will see the default coherency option
set to 'full' which impacts -ve performance. This patch changes coherence
option to buffered which keeps the default behaviour same as old(UEK1).
If an application that does direct i/o needs cache coherency then they can
use mount option 'coherency=full'

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Acked-by: Srinivas Eeda <srinivas.eeda@oracle.com>
(cherry picked from commit 020a20029508d2d7f36470bebd23f053de4b0dbe)