LU-2139 osc: Use SOFT_SYNC to urge server commit

LU-2139 osc: Use SOFT_SYNC to urge server commit

This change adds a BRW page flag, OBD_BRW_SOFT_SYNC. This flag is
intended to urge a server to commit a client's unstable pages to
stable storage. A client will add this flag to any BRW requests while
it is in a state where it has "many" unstable pages pinned in its cache.

The server, upon receiving a page with this flag set, *should* begin
an async commit. The idea being that, with the proactive async commit,
the client's pinned unstable pages will transition into a stable state
faster than they would have otherwise. Although, the server side portion
of this agreement is still unimplemented, so the OBD_BRW_SOFT_SYNC flag
will currently fall on deaf ears.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Change-Id: I08946a5ec6afab390655643f6d3c8320bf010aea
Reviewed-on: http://review.whamcloud.com/4375
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>