Fix possible double free in error path of ixgbe_rxeof.

Fix possible double free in error path of ixgbe_rxeof.

If fmp is not-NULL then the buf is part of the mbuf chain of fmp. So
only m_freem either fmp or buf but clear both values.

Also clear the M_PKTHDR flag if buf aka mp is not the first buffer in the
chain.

Double free found by bluhm@
OK bluhm@ jan@