Fix potential data corruption in iflib

The MP ring may have txq pointers enqueued. Previously, these were
passed to m_free() when IFC_QFLUSH was set. This patch checks for
the value and doesn't call m_free().

Reviewed by: gallatin
Approved by: re (gjb)
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D16882