IB/mlx5: Fix scatter to CQE in DCT QP creation

When scatter to CQE is enabled on a DCT QP it corrupts the mailbox command
since it tried to treat it as as QP create mailbox command instead of a
DCT create command.

The corrupted mailbox command causes userspace to malfunction as the
device doesn't create the QP as expected.

A new mlx5 capability is exposed to user-space which ensures that it will
not enable the feature on DCT without this fix in the kernel.

Fixes: 5d6ff1ba ("IB/mlx5: Support scatter to CQE for DC transport type")
Signed-off-by: Guy Levi <guyle@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>