mlx5: Fix cleanup flow when DMA mapping fails

If DMA mapping fails, the driver cleared the object that holds the
previously DMA mapped pages. Fix this by allocating a new object for
the command that reports back to firmware that pages can't be
supplied.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>