ctcm: avoid wraparound in length of incoming data

Since the receive code should tolerate any incoming garbage, it
should be protected against a potential wraparound when manipulating
length values within incoming data.
block_len is unsigned, so a too large subtraction will cause a
wraparound.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>