IB/srp: disconnect to SRP target before removing SCSI host

There has been a nasty problem upon removing an SRP target when the SRP
target machine crashed accidently without giving back any IB events. In
that case, the admin cannot make use of deleting remote ports for the
purpose of tearing down SRP targets as well as SCSI host. One of the
reasons was a completion on target->done, the other was the invocation
order of srp_disconnect_target() and scsi_remove_host(). Consequence of
the latter was unfortunately hanging forever on device_del(), until the
target machine comes up again after having rebooted.

That symptom is simply reproducible via sysfs. First of all, trigger an
immediate reboot via /proc/sysrq-trigger on the SRP target.

  target# echo b > /proc/sysrq-trigger

After doing that, the Infiniband connection will be completely gone for
several minutes at least. Then on the initiator's side, delete a remote
port by writing 1 to /sys/class/srp_remote_ports/port-*\:1/delete, e.g.:

  initiator# echo 1 > /sys/class/srp_remote_ports/port-6\:1/delete

, where the SRP remote port to be deleted has its number 6.

Then you will see stale SCSI targets remaining despite of rport delete,
which is not expected though. That was resulted from device_del()
hanging forever on destroying SCSI LLD.

The solution consists of two modifications. The first one was already
committed to jejb/for-next. See the commit 55d93898 "IB/srp: send
disconnect request without waiting for CM timewait exit" by Vu Pham
<vu@mellanox.com>. That will prevent from waiting for completion.

The next one, which this commit is saying about, is changing the
invocation order in srp_remove_target(). Call srp_disconnect_target()
before scsi_remove_host(). This change will prevent device_del() from
hanging indefinitely.

This patch is based on the srp-ha-v3.7 tree by Bart Van Assche.
If necessary, I could rebase it on the stable tree.

Signed-off-by: Dongsu Park <dongsu.park@profitbricks.com>
Cc: Sebastian Riemer <sebastian.riemer@profitbricks.com>
Cc: Bart Van Assche <bvanassche@acm.org>