x25: possible skb leak on bad facilities

Originally x25_parse_facilities returned
-1 for an error
 0 meaning 0 length facilities
>0 the length of the facilities parsed.

5ef41308 ("x25: Prevent crashing when parsing bad X.25 facilities") introduced more
error checking in x25_parse_facilities however used 0 to indicate bad parsing
a6331d6f ("memory corruption in X.25 facilities parsing") followed this further for
DTE facilities, again using 0 for bad parsing.

The meaning of 0 got confused in the callers.
If the facilities are messed up we can't determine where the data starts.
So patch makes all parsing errors return -1 and ensures callers close and don't use the skb further.

Reported-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Hendry <andrew.hendry@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>