IB/uverbs: Introduce and use helper functions to copy ah attributes

This patch introduces two helper functions to copy ah attributes
from uverbs to internal ib_ah_attr structure and the other way
during modify qp and query qp respectively.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>