igb: move setting of buffsz out of repeated path in alloc_rx_buffers

buffsz is being repeatedly set when allocaing buffers.  Since this value
should only need to be set once in the function I am moving it out of the
looped portion of the path.

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>