NTB: Xeon Errata Workaround

There is a Xeon hardware errata related to writes to SDOORBELL or
B2BDOORBELL in conjunction with inbound access to NTB MMIO Space, which
may hang the system.  To workaround this issue, use one of the memory
windows to access the interrupt and scratch pad registers on the remote
system.  This bypasses the issue, but removes one of the memory windows
from use by the transport.  This reduction of MWs necessitates adding
some logic to determine the number of available MWs.

Since some NTB usage methodologies may have unidirectional traffic, the
ability to disable the workaround via modparm has been added.

See BF113 in
http://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/xeon-c5500-c3500-spec-update.pdf
See BT119 in
http://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/xeon-e5-family-spec-update.pdf

Signed-off-by: Jon Mason <jon.mason@intel.com>

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