NTB: Add variable number MW, DB CB support code

This is a follow-up to r289208: "Xeon Errata Workaround."

Add logic to support a variable number of memory windows and doorbell
callbacks. This was added to the Linux driver in the "Xeon Errata
Workaround" commit, but I skipped it because it didn't look neccessary
at the time. It is needed for future Haswell split-BAR support, so
bring it in now.

A new tunable was added for if_ntb, 'hw.ntb.max_num_clients'. By
default, it is set to zero -- infer the number of clients from the
number of memory windows available from the hardware. Any other
positive value can specify a different number of clients, limited by the
number of doorbell callbacks available (4 under MSI-X, or 15 (Xeon) or
34 (SoC) under legacy INTx).

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