Add compact scraptchpad protocol for ntb_transport(4).

Previously ntb_transport(4) required at least 6 scratchpad registers,
plus 2 more for each additional memory window. That is too much for some
configurations, where several drivers have to share resources of the same
NTB hardware. This patch introduces new compact version of the protocol,
requiring only 3 scratchpad registers, plus one more for each additional
memory window. The optimization is based on fact that neither of version,
number of windows or number of queue pairs really need more then one byte
each, and window sizes of 4GB are not very useful now. The new protocol
is activated automatically when the configuration is low on scratchpad
registers, or it can be activated explicitly with loader tunable.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.