RDMA/core: Validate the passed in fops for ib_get_ucaps()

RDMA/core: Validate the passed in fops for ib_get_ucaps()

Sashiko pointed out it is not safe to rely only on the devt because
char/block alias so if the user finds a block device with the same dev_t
it can masquerade as a ucap cdev fd.

Test the f_ops to only accept authentic cdevs.

Link: https://patch.msgid.link/r/0-v1-fd9482545e37+1e25-ib_ucaps_fd_ops_jgg@nvidia.com
Cc: stable@vger.kernel.org
Fixes: 61e51682816d ("RDMA/uverbs: Introduce UCAP (User CAPabilities) API")
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>