Fix mlx5en(4) driver to properly call m_defrag().

When the mlx5en(4) driver was converted to using BUSDMA(9) the call to
m_defrag() was moved after the part of the TX routine that strips the
header from the mbuf chain. Before it called m_defrag it first trimmed
off the now-empty mbufs from the start of the chain. This has the side
effect of also removing the head of the chain that has M_PKTHDR set.
m_defrag() will not defrag a chain that does not have M_PKTHDR set,
thus it was effectively never defragging the mbuf chains.

As it turns out, trimming the mbufs in this fashion is unnecessary since
the call to bus_dmamap_load_mbuf_sg doesn't map empty mbufs anyway, so
remove it.

Differential Revision: https://reviews.freebsd.org/D12050
Submitted by: mjoras@
MFC after: 1 week
Sponsored by: Mellanox Technologies