IB/uverbs: Do not use uverbs_cmd_mask in the ioctl path

Instead we are now checking the function pointers directly. Get rid of
both cases in ioctl and drop the nonsense idea that destroy can fail.

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