iser-target: Allocate PI contexts dynamically

This patch converts to allocate PI contexts dynamically in order
avoid a potentially bogus np->tpg_np and associated NULL pointer
dereference in isert_connect_request() during iser-target endpoint
shutdown with multiple network portals.

Also, there is really no need to allocate these at connection
establishment since it is not guaranteed that all the IOs on
that connection will be to a PI formatted device.

We can do it in a lazy fashion so the initial burst will have a
transient slow down, but very fast all IOs will allocate a PI
context.

Squashed:

iser-target: Centralize PI context handling code
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Cc: <stable@vger.kernel.org> # v3.14+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/infiniband/ulp/isert/ib_isert.h

@@ -82,6 +82,12 @@ struct isert_data_buf {
 	enum dma_data_direction dma_dir;
 };
 
+enum {
+	DATA = 0,
+	PROT = 1,
+	SIG = 2,
+};
+
 struct isert_rdma_wr {
 	struct list_head	wr_list;
 	struct isert_cmd	*isert_cmd;
@@ -91,6 +97,7 @@ struct isert_rdma_wr {
 	int			send_wr_num;
 	struct ib_send_wr	*send_wr;
 	struct ib_send_wr	s_send_wr;
+	struct ib_sge		ib_sg[3];
 	struct isert_data_buf	data;
 	struct isert_data_buf	prot;
 	struct fast_reg_descriptor *fr_desc;