er_walk may return a negative error value, handle this properly
instead of treating it as a huge number of scatter-gather elements.
    
Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
R³»