sfxge: split sfxge_tx_qdpl_put() into *_locked() and *_unlocked()

It simplifies understanding of the sfxge_tx_packet_add() logic and
avoids passing of 'locked' to called function.

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D2547