kmsg - properly print over-long continuation lines

Reserve PREFIX_MAX bytes in the LOG_LINE_MAX line when buffering a
continuation line, to be able to properly prefix the LOG_LINE_MAX
line with the syslog prefix and timestamp when printing it.

Reported-By: Dave Jones <davej@redhat.com>
Signed-off-by: Kay Sievers <kay@vrfy.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>