iflib: netmap: add support for NS_MOREFRAG

The NS_MOREFRAG flag can be set in a netmap slot to represent a
multi-fragment packet. Only the last fragment of a packet does
not have the flag set. On TX rings, the flag may be set by the
userspace application. The kernel will look at the flag and use it
to properly set up the NIC TX descriptors.
On RX rings, the kernel may set the flag if the packet received
was split across multiple netmap buffers. The userspace application
should look at the flag to know when the packet is complete.

Submitted by: rajesh1.kumar_amd.com
Reviewed by: vmaffione
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27799

(cherry picked from commit aceaccab659c9eb846fb21ff99be34434a9616c7)