ixgbe: Update method used for determining descriptor count for an skb

This patch updates the current methods used for determining if we have
enough space to transmit a given skb.  The current method is quite wasteful
as it has us go through and determine how each page is going to be broken
up.  That only needs to be done if pages are larger than our maximum data
per TXD.  As such I have wrapped that in a page size check.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>