[IPSEC]: Return EOVERFLOW when output sequence number overflows

Previously we made it an error on the output path if the sequence number
overflowed.  However we did not set the err variable accordingly.  This
patch sets err to -EOVERFLOW in that case.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>