The ALSA proc handler allows currently the write in the unlimited size
until kmalloc() fails.  But basically the write is supposed to be only
for small inputs, mostly for one line inputs, and we don't have to
handle too large sizes at all.  Since the kmalloc error results in the
kernel warning, it's better to limit the size beforehand.

This patch adds the limit of 16kB, which must be large enough for the
currently existing code.

Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Takashi Iwai <tiwai@suse.de>