NTB: Remap MSI-X messages over available slots

Remap MSI-X messages over available slots rather than falling back to
legacy INTx when fewer MSI-X slots are available than were requested.

N.B. the Linux driver does *not* do this.

To aid in testing, a tunable 'hw.ntb.force_remap_mode' has been added.
It defaults to off (0). When the tunable is enabled and sufficient
slots were available, the driver restricts the number of slots by one
and remaps the MSI-X messages over the remaining slots.

In case this is actually not okay (as I don't yet have access to this
hardware to test), a tunable 'hw.ntb.prefer_intx_to_remap' has been
added. It defaults to off (0). When the tunable is enabled and fewer
slots are available than requested, fall back to legacy INTx mode rather
than attempting to remap MSI-X messages.

Suggested by: jhb
Reviewed by: jhb (earlier version)
Sponsored by: EMC / Isilon Storage Division