MFC: phk's bus_{read,write}_* macros.  These are slightly different than -current because rman is opaque in RELENG_6, but are 100% API compatible. If they prove to be too slow, we may be able to make rman non-opaque to mitigate it.  This should help porting drivers from -current into -stable.
MFC: phk's bus_{read,write}_* macros.  These are slightly different than
-current because rman is opaque in RELENG_6, but are 100% API compatible.
If they prove to be too slow, we may be able to make rman non-opaque to
mitigate it.  This should help porting drivers from -current into
-stable.

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