xen-netback: don't use last request to determine minimum Tx credit
xen-netback: don't use last request to determine minimum Tx credit

The last from guest transmitted request gives no indication about the
minimum amount of credit that the guest might need to send a packet
since the last packet might have been a small one.

Instead allow for the worst case 128 KiB packet.

This is part of XSA155.

CC: 4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org
Reviewed-by: Wei Liu <a5240e9e4c4d742e3a4232ca749f78defd45d722@citrix.com>
Signed-off-by: David Vrabel <b9d45bd1f671e508cd8daa63dfc3cabb597562df@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <da3a51b335cef0eb0e2c329c5ef6bcd6acef687a@oracle.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs