iser-target: introduce fast memory registration mode (FRWR)

This model was introduced in 00f7ec36 "RDMA/core: Add memory
management extensions support" and works when the IB device
supports the IB_DEVICE_MEM_MGT_EXTENSIONS capability.

Upon creating the isert device, ib_isert will test whether the HCA
supports FRWR. If supported then set the flag and assign
function pointers that handle fast registration and deregistration
of appropriate resources (fast_reg descriptors).

When new connection coming in, ib_isert will check frwr flag and
create frwr resouces, if fail to do it will switch back to
old model of using global dma key and turn off the frwr support.

Registration is done using posting IB_WR_FAST_REG_MR to the QP and
invalidations using posting IB_WR_LOCAL_INV.

Signed-off-by: Vu Pham <vu@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>