net: sysctl: do not reserve an extra char in dump_cpumask temporary buffer

net: sysctl: do not reserve an extra char in dump_cpumask temporary buffer

When computing the length we'll be able to use out of the buffers, one
char is removed from the temporary one to make room for a newline. It
should be removed from the output buffer length too, but in reality this
is not needed as the later call to scnprintf makes sure a null char is
written at the end of the buffer which we override with the newline.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>