RDMA/uverbs: Check for NULL driver methods for every write call

Add annotations to the uverbs_api structure indicating which driver
methods are called by the implementation. If the required method
is NULL the write API will be not be callable.

This effectively duplicates the cmd_mask system, however it does it by
expressing invariants required by the core code, not by delegating
decision making to the driver. This is another step toward eliminating
cmd_mask.

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