IB/iser: Enable iser when FMRs are not supported

Reuse the "SG unaligned for FMR" driver flow to make the initiator
functional when running over driver instance which doesn't support
FMRs, such as a mlx4 virtual function.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Alex Tabachnik <alext@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>