zfs: use atomic_load_64 to read atomic variable in dmu_object_alloc_impl

As long as we support ZFS on 32-bit platforms we should do this for all
64-bit variables that are modified in a lockless fashion using atomic
operations. Otherwise, there is a risk of a reading a torn value.

Here is a rationale for why I am doing this in dmu_object_alloc_impl:

• it's very recent code
• the code deals with object IDs and a number of objects in a file system can overflow 32 bits
• incorrect allocation of an object ID may result in hard to debug problems
• fixing all plain reads of 64-bit atomic variables is not a trivial undertaking to do in one shot, so I chose to do it incrementally

MFC after: 3 weeks
X-MFC after: r353301, r353176