igb: Do not parse past IP header on fragments beyond the first

igb: Do not parse past IP header on fragments beyond the first

This change makes it so that only the first fragment in a series of fragments
will have the L4 header pulled.  Previously we were always pulling the L4
header as well and in the case of UDP this can harm performance since only the
first fragment will have the header, the rest just contain data which should
be left in the paged portion of the packet.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>