net: stmmac: WARN if tx_skbuff entries are reused before cleared

The current code assumes that a tx_skbuff entry has been cleared
by stmmac_tx_clean() before stmmac_xmit()/stmmac_tso_xmit()
assigns a new skb to that entry. However, since we never check
the current value before overwriting it, it is theoretically
possible that a non-NULL value is overwritten.

Add WARN_ONs to verify that each entry in tx_skbuff is NULL
before it is assigned a new value.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>