RDMA/core Remove uverbs_ex_cmd_mask

No driver sets it, and the core code sets a maximum mask, simply remove
it.

Disabled operations are now handled either by having a NULL ops pointer,
or by having the common driver callbacks check for unsupported extended
attributes.

Link: https://lore.kernel.org/r/9-v1-caa70ba3d1ab+1436e-ucmd_mask_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>