IB/hfi1: Use after free race condition in send context error path

A pio send egress error can occur when the PSM library attempts to
to send a bad packet.  That issue is still being investigated.

The pio error interrupt handler then attempts to progress the recovery
of the errored pio send context.

Code inspection reveals that the handling lacks the necessary locking
if that recovery interleaves with a PSM close of the "context" object
contains the pio send context.

The lack of the locking can cause the recovery to access the already
freed pio send context object and incorrectly deduce that the pio
send context is actually a kernel pio send context as shown by the
NULL deref stack below:

[<ffffffff8143d78c>] _dev_info+0x6c/0x90
[<ffffffffc0613230>] sc_restart+0x70/0x1f0 [hfi1]
[<ffffffff816ab124>] ? __schedule+0x424/0x9b0
[<ffffffffc06133c5>] sc_halted+0x15/0x20 [hfi1]
[<ffffffff810aa3ba>] process_one_work+0x17a/0x440
[<ffffffff810ab086>] worker_thread+0x126/0x3c0
[<ffffffff810aaf60>] ? manage_workers.isra.24+0x2a0/0x2a0
[<ffffffff810b252f>] kthread+0xcf/0xe0
[<ffffffff810b2460>] ? insert_kthread_work+0x40/0x40
[<ffffffff816b8798>] ret_from_fork+0x58/0x90
[<ffffffff810b2460>] ? insert_kthread_work+0x40/0x40

This is the best case scenario and other scenarios can corrupt the
already freed memory.

Fix by adding the necessary locking in the pio send context error
handler.

Cc: <stable@vger.kernel.org> # 4.9.x
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>