phonet: some signedness bugs

Dan Rosenberg pointed out that there were some signed comparison bugs
in the phonet protocol.

http://marc.info/?l=full-disclosure&m=129424528425330&w=2



The problem is that we check for array overflows but "protocol" is
signed and we don't check for array underflows.  If you have already
have CAP_SYS_ADMIN then you could use the bugs to get root, or someone
could cause an oops by mistake.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>