igb: prevent skb_over panic w/ mtu smaller than 1K

A panic has been observed with frame sizes smaller than 1K.  This has been
root caused to the hardware spanning larger frames across multiple buffers
and then reporting the original frame size in the first descriptor.  To
prevent this we can enable set the LPE bit which in turn will restrict
packet sizes to those set in the RLPML register.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>