git: igb: Free tx mbufs proactively.

[Sepherosa Ziehau]

commit 620b9679515f9db3801f83d855667df59c49a669
Author: Sepherosa Ziehau <sephe at dragonflybsd.org>
Date:   Sun Oct 29 03:01:14 2017 +0800

    igb: Free tx mbufs proactively.
    
    For 82575, which is earliest product of this product line, the RS
    bit is set on every packet's last TX desc by default, since in the
    'head write back' mode, the content of TDH register does not move,
    if the content of the 'header', which is memory based, does not
    move.  It is still allowed to reduce the density of RS TX descs,
    which will be useful for workloads w/o using sendfile.
    
    This is preparation for the dillon's upcoming sendfile patch.

Summary of changes:
 sys/dev/netif/igb/if_igb.c | 213 +++++++++++++++++++++++++++++++++++----------
 sys/dev/netif/igb/if_igb.h |  10 ++-
 2 files changed, 175 insertions(+), 48 deletions(-)

http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/620b9679515f9db3801f83d855667df59c49a669


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