RDMA/uverbs: Add helpers to mark uapi functions as unsupported

We have many cases where parts of the uapi are not supported in a driver,
needs a certain protocol, or whatever. It is best to reflect this directly
into the struct uverbs_api when it is built so that everything is simply
blocked off, and future introspection can report a proper supported list.

This is done by adding some additional helpers to the definition list
language that disable objects based on a 'supported' call back, and a
helper that disables based on a NULL struct ib_device function pointer.

Disablement is global. For instance, if a driver disables an object then
everything connected to that object is removed, including core methods.

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