This change makes it so that we pull the timestamp from the fragment before
we add it to the skb.  By doing this we can avoid a possible issue in which
the fragment can possibly be less than IGB_RX_HDR_LEN due to the timestamp
being pulled after the copybreak check.

While making this change I realized we could also pull the rest of the
igb_pull_tail function into igb_add_rx_frag since in the case of igb,
unlike ixgbe, we are able to unmap the entire buffer before calling
add_rx_frag so merging the two allows for sharing of code between the two
merged functions.
Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>