vhost: correctly check the return value of translate_desc() in log_used()
When fail, translate_desc() returns negative value, otherwise the
number of iovs. So we should fail when the return value is negative
instead of a blindly check against zero.
Detected by CoverityScan, CID# 1442593: Control flow issues (DEADCODE)
Fixes: cc5e71075947 ("vhost: log dirty page correctly")
Acked-by: Michael S. Tsirkin <mst at redhat.com>
Reported-by: Stephen Hemminger <stephen at networkplumber.org>
Signed-off-by: Jason Wang <jasowang at redhat.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
UnifiedSplitRaw
