MFC r346570, r348440

r346570:
cxgbe/t4_tom: Add a "TCB history" feature that samples hardware state
for a tid and maintains a running history of some interesting events.

Service TCP_INFO queries from the history when the tid is being tracked
there.

r348440:
cxgbe/t4_tom: Do not attempt to look up entries in the TCB history if
it hasn't been initialized.

This fixes a bug in r346570 that could cause a panic when servicing
TCP_INFO for offloaded connections.

Sponsored by: Chelsio Communications