qlcnic: Add null check after calling netdev_alloc_skb

qlcnic: Add null check after calling netdev_alloc_skb

The function qlcnic_dl_lb_test() currently calls netdev_alloc_skb()
without checking afterwards that the allocation succeeded. Fix this by
checking if the skb is NULL and returning an error in such a case.
Breaking out of the loop if the skb is NULL is not correct as no error
would be reported to the caller and no message would be printed for the
user.

Cc: David S. Miller <davem@davemloft.net>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Tom Seewald <tseewald@gmail.com>
Link: https://lore.kernel.org/r/20210503115736.2104747-26-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>