net/enic: streamline mbuf handling in Tx path

net/enic: streamline mbuf handling in Tx path

The list of mbufs held by the driver on Tx was allocated in chunks
(a hold-over from the enic kernel mode driver). The structure used
next pointers across chunks which led to cache misses.

Allocate the array used to hold mbufs in flight on Tx with
rte_zmalloc_socket(). Remove unnecessary fields from the structure
and use head and tail pointers instead of next pointers.

Signed-off-by: John Daley <johndale@cisco.com>