nfq: consistent nfq_snprintf_xml() return value with snprintf()

nfq: consistent nfq_snprintf_xml() return value with snprintf()

With this patch, nfq_snprintf_xml() returns the number of characters
printed. If the output was truncated, then the return value is the
number of characters that would have been written if enough space
had been available. This makes nfq_snprintf_xml() consistent with
the behaviour of snprintf().

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>