MFC r317702, r317847, r318307

r317702:
cxgbe(4): Support routines for Tx traffic scheduling.

• Create a new file, t4_sched.c, and move all of the code related to traffic management from t4_main.c and t4_sge.c to this file.
• Track both Channel Rate Limiter (ch_rl) and Class Rate Limiter (cl_rl) parameters in the PF driver.
• Initialize all the cl_rl limiters with somewhat arbitrary default rates and provide routines to update them on the fly.
• Provide routines to reserve and release traffic classes.

r317847:
cxgbe(4): The Tx scheduler initialization either works or doesn't. It
doesn't need a refresh in either case.

r318307:
cxgbe(4): Avoid an out of bounds access when an attempt to unbind a tx
queue from a traffic class fails.

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