Don't omit m_dup() for non-writeable mbufs that need checksum calculation

If the driver is not active or link is down the packet could remain
non-writeable. This commit makes all mbufs enqueued to the driver's
ring buffer to have correct attributes.

Pointed out by: wma
Reviewed by: wma
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D5800