RDMA/uverbs: Use an unambiguous errno for method not supported

Returning EOPNOTSUPP is problematic because it can also be
returned by the method function, and we use it in quite a few
places in drivers these days.

Instead, dedicate EPROTONOSUPPORT to indicate that the ioctl framework
is enabled but the requested object and method are not supported by
the kernel. No other case will return this code, and it lets userspace
know to fall back to write().

grep says we do not use it today in drivers/infiniband subsystem.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>