selftests: ublk: add test to verify that feat_map is complete

selftests: ublk: add test to verify that feat_map is complete

Add a test that verifies that the currently running kernel does not
report support for any features that are unrecognized by kublk. This
should catch cases where features are added without updating kublk's
feat_map accordingly, which has happened multiple times in the past (see
[1], [2]).

Note that this new test may fail if the test suite is older than the
kernel, and the newer kernel contains a newly introduced feature. I
believe this is not a use case we currently care about - we only care
about newer test suites passing on older kernels.

[1] https://lore.kernel.org/linux-block/20250606214011.2576398-1-csander@purestorage.com/t/#u
[2] https://lore.kernel.org/linux-block/2a370ab1-d85b-409d-b762-f9f3f6bdf705@nvidia.com/t/#m1c520a058448d594fd877f07804e69b28908533f

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>