s390/qeth: use memory reserves in TX slow path

When falling back to an allocation from the HW header cache, check if
the skb is eligible for using memory reserves.
This only makes a difference if the cache is empty and needs to be
refilled.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>