RDMA/verbs: Store the write/write_ex uapi entry points in the uverbs_api

Bringing all uapi entry points into one place lets us deal with them
consistently. For instance the write, write_ex and ioctl paths can be
disabled when an API is not supported by the driver.

This will replace the uverbs_cmd_table static arrays.

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