vdpa/mlx5: Fix uninitialised variable in core/mr.c

vdpa/mlx5: Fix uninitialised variable in core/mr.c

If the kernel is unable to allocate memory for the variable dmr then
err will be returned without being set. Set err to -ENOMEM in this
case.

Fixes: 94abbccdf291 ("vdpa/mlx5: Add shared memory registration code")
Addresses-Coverity: ("Uninitialized variables")
Signed-off-by: Alex Dewar <alex.dewar@gmx.co.uk>
Link: https://lore.kernel.org/r/20200806185625.67344-1-alex.dewar@gmx.co.uk
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Eli Cohen <eli@mellanox.com>