NTB: MFV 49793889: Rename Variables for NTB-RP

Many variable names in the NTB driver refer to the primary or secondary
side. However, these variables will be used to access the reverse case
when in NTB-RP mode. Make these names more generic in anticipation of
NTB-RP support.

Authored by: Jon Mason
Obtained from: Linux
Sponsored by: EMC / Isilon Storage Division