IB/iser: Use IB_WR_REG_MR_INTEGRITY for PI handover

Using this new API reduces iSER code complexity.
It also reduces the maximum number of work requests per task and the need
of dealing with multiple MRs (and their registrations and invalidations)
per task. It is done by using a single WR and a special MR type
(IB_MR_TYPE_INTEGRITY) for PI operation.

The setup of the tested benchmark:
 - 2 servers with 24 cores (1 initiator and 1 target)
 - 24 target sessions with 1 LUN each
 - ramdisk backstore
 - PI active

Performance results running fio (24 jobs, 128 iodepth) using
write_generate=0 and read_verify=0 (w/w.o patch):

bs      IOPS(read)        IOPS(write)
----    ----------        ----------
512     1236.6K/1164.3K   1357.2K/1332.8K
1k      1196.5K/1163.8K   1348.4K/1262.7K
2k      1016.7K/921950    1003.7K/931230
4k      662728/600545     595423/501513
8k      385954/384345     333775/277090
16k     222864/222820     170317/170671
32k     116869/114896     82331/82244
64k     55205/54931       40264/40021

Using write_generate=1 and read_verify=1 (w/w.o patch):

bs      IOPS(read)        IOPS(write)
----    ----------        ----------
512     1090.1K/1030.9K   1303.9K/1101.4K
1k      1057.7K/904583    1318.4K/988085
2k      965226/638799     1008.6K/692514
4k      555479/410151     542414/414517
8k      298675/224964     264729/237508
16k     133485/122481     164625/138647
32k     74329/67615       80143/78743
64k     35716/35519       39294/37334

We get performance improvement at all block sizes.
The most significant improvement is when writing 4k bs (almost 30% more
iops).

Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>