xfrm_alloc_spi will always succeed if minspi < maxspi, even if
minspi + 1 == maxspi.  If the range is already occupied this
will obviously lead to breakage.

Of course this is very unlikely to occur in reality due to the
size of the range.  Although with IPCOMP it might actually happen
on a very large server.

The fix is obivous.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@redhat.com>