[infiniband] Make qkey and rate optional parameters to ib_post_send()

[infiniband] Make qkey and rate optional parameters to ib_post_send()

The queue key is stored as a property of the queue pair, and so can
optionally be added by the Infiniband core at the time of calling
ib_post_send(), rather than always having to be specified by the
caller.

This allows IPoIB to avoid explicitly keeping track of the data queue
key.