sparc64: Disable DAX flow control

sparc64: Disable DAX flow control

Orabug: 25997202

The flow control feature of DAX only works for output buffers
and it doesn't appear to be possible to calculate the exact
size in bytes of an input buffer. So we have no way to bound
the amount of data that DAX reads as input. This has correctness
and security implications, so until we figure out something better
to do, the temporary workaround is to disable flow control completely
and fall back to 4Mb virtual page backed dax_alloc memory, which
will allow page boundaries to limit the size of all buffers.  A new
module parameter "flow_enable" is provided to allow this decision
to be reverted at module load time if needed.

Signed-off-by: Rob Gardner <rob.gardner@oracle.com>
Reviewed-by: Jonathan Helman <jonathan.helman@oracle.com>
Reviewed-by: Sanath Kumar <sanath.s.kumar@oracle.com>
Signed-off-by: Allen Pais <allen.pais@oracle.com>