cxgbetool(8): Add a subaction (tcbrss <n>) that can be used with "pass"
action to distribute traffic using the half of the VI's RSS indirection
table.

The value specified should either be the start of the VI's RSS slice
(available at dev.<ifname>.<inst>.rss_base since r339700) or the
midpoint (rss_base + rss_size/2). The traffic that hits the filter will
use the first or second half of the indirection table respectively.
The indirection table can be populated in different ways to achieve
different kinds of traffic/load distributions. For example, r339749
allows a netmap interface to have half the rx queues in the first half
of the table and the rest in the other.

Sponsored by: Chelsio Communications