IB/uverbs: Use __aligned_u64 for uapi headers

This has no impact on the structure layout since these structs already
have their u64s already properly aligned, but it does document that we
have this requirement for 32 bit compatibility.

Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>