x25: Prevent crashing when parsing bad X.25 facilities

Now with improved comma support.

On parsing malformed X.25 facilities, decrementing the remaining length
may cause it to underflow.  Since the length is an unsigned integer,
this will result in the loop continuing until the kernel crashes.

This patch adds checks to ensure decrementing the remaining length does
not cause it to wrap around.

Signed-off-by: Dan Rosenberg <drosenberg@vsecurity.com>
Signed-off-by: David S. Miller <davem@davemloft.net>