Export dbuf_* symbols

Export these symbols so they may be used by other ZFS consumers
besides the ZPL.

Remove three stale prototype definites from dbuf.h. The actual
implementations of these functions were removed/renamed long ago.

It would be good in the long term to remove the existing pragmas
we inherited from Solaris and simply use the dbuf_* names.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>