MFC: r283048
MFC: r283048

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.

Sponsored by:   Solarflare Communications, Inc.

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