NTB: MFV 948d3a65: 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

Authored by:	Jon Mason
Obtained from:	Linux
Sponsored by:	EMC / Isilon Storage Division

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