scsi: message: mptlan: Fix use after free bug in mptlan_remove() due to race condition

In the Linux kernel, the following vulnerability has been resolved:

scsi: message: mptlan: Fix use after free bug in mptlan_remove() due to race condition

mptlan_probe() calls mpt_register_lan_device() which initializes the
&priv->post_buckets_task workqueue. A call to
mpt_lan_wake_post_buckets_task() will subsequently start the work.

During driver unload in mptlan_remove() the following race may occur:

CPU0 CPU1

|mpt_lan_post_receive_buckets_work()
mptlan_remove() |
free_netdev() |
kfree(dev); |
|
| dev->mtu
| //use

Fix this by finishing the work prior to cleaning up in mptlan_remove().

[mkp: we really should remove mptlan instead of attempting to fix it]