enic: Add support for MTU change via port profile on a dynamic vnic

enic driver gets MTU change notifications for MTU changes in the
port profile associated to a dynamic vnic. This patch adds support
in enic driver to set new MTU on the dynamic vnic and dynamically
adjust its buffers with new MTU size in response to such notifications.

Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: David Wang <dwang2@cisco.com>
Signed-off-by: Christian Benvenuti <benve@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>