igb: Don't use NETDEV_FRAG_PAGE_MAX_SIZE in descriptor calculation

igb: Don't use NETDEV_FRAG_PAGE_MAX_SIZE in descriptor calculation

Orabug: 21792102

This change updates igb so that it will correctly perform the descriptor
count calculation.  Previously it was taking NETDEV_FRAG_PAGE_MAX_SIZE
into account with isn't really correct since a different value is used to
determine the size of the pages used for TCP.  That is actually determined
by SKB_FRAG_PAGE_ORDER.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 2ee52ad4962b32797bac33fa29ec8159e64c4ee3)
Signed-off-by: Brian Maly <brian.maly@oracle.com>