networking: fix potential overflow in ife_print6()

devname is used by fscanf to store a string specified by '%20s'.

Extract from the man for the '%s' specifier:
Matches a sequence of non-white-space characters; the next pointer must be a
pointer to character array that is long enough to hold the input sequence and
the terminating null byte ('\0'), which is added automatically. The input
string stops at white space or at the maximum field width, whichever occurs
first.

Hence, the right length is 20 + 1 for the '\0'.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>