sfxge: fix overflow queue freeze

If TxQ lock is obtained, deferred packet list shold be serviced even if
the packet addition fails because of overflow.

Without the patch freeze happens if:

• queue is not blocked (i.e. completion does not trigger unblock and service)
• put-list overflow (1024 entries)
• sfxge_tx_packet_add() acquires TxQ lock just as it is released it in sfxge_tx_qdpl_service() on the second CPU but before pending check
• sfxge_tx_packet_add() swizzles put-list to get-list, fails because of non-tcp get-list overflow and returns without packet list service
• sfxge_tx_qdpl_service() on the second CPU checks that there are no pending packets in the put-list and returns

Other possible solution is to guaranee that maximum length of the put-list
is less than maximum length of any get-list.

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D2562