cxgbe/t4_tom: Change stid allocation strategy to be more IPv6 friendly.

A hardware IPv6 server needs 2 consecutive stids (server tids) starting
from a 2-aligned stid whereas an IPv4 server needs only 1 stid without
any constraint. The allocator used to grab the first free stid(s) for
both but this can fragment the stid space leaving nothing suitable for
IPv6 even when lots of stids are available. Change the allocator to
prefer stids for IPv4 from the ones that cannot be used for IPv6.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D47042