osc/rdma: ensure fragment headers and the packed datatype are 8-byte aligned.

The datatype unpacking code assumes that the packed datatype buffer has the
same alignment as an OPAL_PTRDIFF_TYPE. This was not enforced by the rdma
one-sided component. I changed the ordering and sized of various osc/rdma
headers to ensure their sizes are a multiple of 8-bytes and modified the
fragment allocation call to ensure all headers are 8-byte aligned. While
not the cleanest way to handle this situation it should resolve the issue.

Fixes #4315

cmr=v1.7.5:reviewer=jsquyres
osc/rdma: use OPAL_ALIGN macro

cmr=v1.7.5:ticket=#4357