net/sfc: support EF100 Tx override prefix

net/sfc: support EF100 Tx override prefix

Add internal mbuf dynamic flag and field to request EF100 native
Tx datapath to use Tx prefix descriptor to override egress m-port.
Overriding egress m-port is necessary on representor Tx burst
so that the packet will reach corresponding VF.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>
Reviewed-by: Ivan Malov <ivan.malov@oktetlabs.ru>