avf: Handle chain buffer in TX properly

avf: Handle chain buffer in TX properly

For chain buffer, need to traverse b->next_buffer to transmit all buffers
in the chain. Only set EOP at the last descriptor in the chain to signal
this is a chain descriptor.

Introduce slow path to handle ring wrap. This is needed because chain
buffer may consist of multiple pieces and it may span from near the end of
the ring to the beginning of the ring.

Type: fix
Ticket: VPP-1843

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: Id7c872f3e39e09f3566aa63f3cdba8f40736d508
(cherry picked from commit f7ab7b2d9bc0f42c1e766d22d49dd0dc4f28abb6)