vmxnet3: add support for RSS kernel option

We observe at least one problem: if a UDP socket is connect(2)-ed, then a
received packet that matches the connection cannot be matched to the
corresponding PCB because of an incorrect flow ID. That was oberved for DNS
requests from the libc resolver. We got this problem because FreeBSD
r343291 enabled code that can set rsstype of received packets to values
other than M_HASHTYPE_OPAQUE_HASH. Earlier that code was under 'ifdef
notyet'.

The essence of this change is to use the system-wide RSS key instead of
some historic hardcoded key when the software RSS is enabled and it is
configured to use Toeplitz algorithm (the default).
In all other cases, the driver reports the opaque hash type for received
packets while still using Toeplitz algorithm with the internal key.

PR: 242890
Reviewed by: pkelsey
Sponsored by: Panzura
Differential Revision: https://reviews.freebsd.org/D23147