IB/iser: Enable SG clustering

IB/iser: Enable SG clustering

iser is perfectly capable supporting SG clustering as it translates
the SG list to a page vector. Enabling SG clustering can dramatically
reduce the number of SG elements, which doesn't make much of a difference
at this point, but with arbitrary SG list support, reducing the
number of SG elements can benefit greatly as as it would reduce
the length of the HW descriptors array.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>