scsi: be2iscsi: Fix for crash in beiscsi_eh_device_reset

scsi: be2iscsi: Fix for crash in beiscsi_eh_device_reset

System crashes when sg_reset is executed in a loop.
CPU: 13 PID: 7073 Comm: sg_reset Tainted: G            E   4.8.0-rc1+ #4
RIP: 0010:[<ffffffffa0825370>]  [<ffffffffa0825370>]
beiscsi_eh_device_reset+0x160/0x520 [be2iscsi]
Call Trace:
[<ffffffff814c7c77>] ? scsi_host_alloc_command+0x47/0xc0
[<ffffffff814caafa>] scsi_try_bus_device_reset+0x2a/0x50
[<ffffffff814cb46e>] scsi_ioctl_reset+0x13e/0x260
[<ffffffff814ca477>] scsi_ioctl+0x137/0x3d0
[<ffffffffa05e4ba2>] sg_ioctl+0x572/0xc20 [sg]
[<ffffffff8123f627>] do_vfs_ioctl+0xa7/0x5d0

The accesses to beiscsi_io_task is being protected in device reset handler
with frwd_lock but the freeing of task can happen under back_lock.

Hold the reference of iscsi_task till invalidation completes.
This prevents use of ICD when invalidation of that ICD is being processed.
Use frwd_lock for iscsi_tasks looping and back_lock to access
beiscsi_io_task structures.

Rewrite mgmt_invalidation_icds to handle allocation and freeing of IOCTL
buffer in one place.

Signed-off-by: Jitendra Bhivare <jitendra.bhivare@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>