[IB] uverbs: Implement more commands

Add kernel support for userspace calling poll CQ, request CQ
notification, post send, post receive, post SRQ receive, create AH and
destroy AH commands.  These commands allow us to support userspace
verbs for devices that can't perform these operations directly from
userspace (eg the PathScale HCA).

Signed-off-by: Roland Dreier <rolandd@cisco.com>