cxgbe(4): Allow t4_tom to be unloaded safely.

• Disable IFCAP_TOE automatically on all ifnets on all adapters during unload. This is user-friendly and avoids panics due to stale ifnet state after t4_tom is unloaded.
• Do not allow unload if tids are in use by the TOE on any adapter.

Reported by: Bimal Abraham @ Chelsio
Sponsored by: Chelsio Communications

(cherry picked from commit 9ba8670a8b175de79ea087688f51595b4f2db862)