NTB: Flesh out the rest of the xeon_setup_b2b_mw changes

Move all Xeon secondary register setup to the setup_b2b_mw routine. We
use subroutines to make it a bit less wordy than the Linux version.

Adds a new tunable, 'hw.ntb.b2b_mw_share'. By default, it is off
(zero). If both sides enable it (any non-zero value), the NTB driver
attempts to use only half of a memory window for remote register MMIO
access.

This is still part of the large Linux rewrite (e26a5843).

Authored by: Allen Hubbe
Obtained from: Linux (e26a5843) (Dual BSD/GPL driver)
Sponsored by: EMC / Isilon Storage Division