RDMA/uverbs: Add a simple iterator interface for reading the command

Several methods have a command with a trailing flex array, and they
all open code some extraction scheme. Centralize this into a simple
iterator API.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>