xen/netback: Always pull through PKT_PROT_LEN bytes into the linear part of an skb

xen/netback: Always pull through PKT_PROT_LEN bytes into the linear part of an skb

Previously PKT_PROT_LEN would only have an effect on the first
fragment.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
xen/netback: try to pull a minimum of 72 bytes into the skb data area

... when receiving a packet into netback.  The previous number, 64,
tended to place a fragment boundary in the middle of the TCP header
options and led to unnecessary fragmentation in Windows <-> Windows
networking.

xen/netback: Re-define PKT_PROT_LEN to be bigger.

Re-define PKT_PROT_LEN to be big enough to handle maximal IPv4 and TCP
options and phrase the definition so that it's reasonably obvious
that's what it's for.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@novell.com>