This can be used with cards that require the driver to construct and
parse packet headers manually.  Headers are optionally handled
out-of-line from the packet payload, since some such cards will split
received headers into a separate ring buffer.

Not all Infiniband cards have embedded subnet management agents.
Split out the code that communicates with such an embedded SMA into a
separate ib_smc.c file, and have drivers call ib_smc_update()
explicitly when they suspect that the answers given by the embedded
SMA may have changed.