cxgbe/tom: Fix potential leak in t4_aiotx_process_job.

The mbuf allocated could be a chain and must be freed with m_freem.

Reviewed by: jhb@
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D29579

(cherry picked from commit bf5057691bb0d1160d729772cdb4c449e366f5b4)