xen-netback: use jiffies_64 value to calculate credit timeout

xen-netback: use jiffies_64 value to calculate credit timeout

time_after_eq() only works if the delta is < MAX_ULONG/2.

For a 32bit Dom0, if netfront sends packets at a very low rate, the time
between subsequent calls to tx_credit_exceeded() may exceed MAX_ULONG/2
and the test for timer_after_eq() will be incorrect. Credit will not be
replenished and the guest may become unable to send packets (e.g., if
prior to the long gap, all credit was exhausted).

Use jiffies_64 variant to mitigate this problem for 32bit Dom0.

Suggested-by: Jan Beulich <[email protected]>
Signed-off-by: Wei Liu <[email protected]>
Reviewed-by: David Vrabel <[email protected]>
Cc: Ian Campbell <[email protected]>
Cc: Jason Luan <[email protected]>
Signed-off-by: David S. Miller <[email protected]>