RDMA/nldev: Add explicit pad attribute

Add a specific RDMA_NLDEV_ATTR_PAD attribute to be used for 64b
attribute padding.  To preserve the ABI, make this attribute equal to
RDMA_NLDEV_ATTR_UNSPEC, which has a value of 0, because that has been
used up until now as the pad attribute.

Change all the previous use of 0 as the pad with this
new enum.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>