Fix a double-free of mbufs in rx_ixgbe_discard().

fmp->buf at the free point is already part of the chain being freed,
so double-freeing is counter-productive.

Submitted by:	Marc De La Gueronniere <mdelagueronniere@verisign.com>
MFC after:	1 week
Sponsored by:	Verisign, Inc.
The error bits are not valid with EOP=0; so intermediary fragments should
not be discarded.

Submitted by:	Marc De La Gueronniere <mdelagueronniere@verisign.com>
MFC after:	1 week
Sponsored by:	Verisign, Inc.