] icmp: fix icmp_unreach() comment.

ICMP_PARAMETERPROB is handled by icmp_unreach(); This patch adds
ICMP_PARAMETERPROB to the list of ICMP message types handled by icmp_unreach().

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>