nvme: Explicitly align struct nvme_command on an 8 byte boundary

This was already true for most architectures due to uint64_t structure
members. However, i386 is special in that it only requires 4 byte
alignment for uint64_t. As a result, casts from struct nvme_command
to struct nvmf_fabric_cmd were raising a "cast increases alignment"
warning on i386. Explicitly aligning struct nvme_command pacifies
this warning on i386.

Reported by: rscheff
Sponsored by: Chelsio Communications