Target/iser: Use Fastreg only if device supports signature

Fastreg is mandatory for signature, so if the
device doesn't support it we don't need to use it.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

diff --cc drivers/infiniband/ulp/isert/ib_isert.c
Simple merge