s390/qeth: overhaul L3 IP address dump code

The current code that dumps the RXIP/VIPA/IPATO addresses via sysfs
first checks whether the buffer still provides sufficient space to hold
another formatted address.
But the maximum length of an formatted IPv4 address is 15 characters,
not 12. So we underestimate the max required length and if the buffer
was previously filled to _just_ the right level, a formatted address can
end up being truncated.

Revamp these code paths to use the _actually_ required length of the
formatted IP address, and while at it suppress a gratuitous newline.

Also use scnprintf() to format the output. In case of a truncation, this
would allow us to return the number of characters that were actually
written.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>