dm writecache: fix the maximum number of arguments

commit 67aa3ec3dbc43d6e34401d9b2a40040ff7bb57af upstream.

Advance the maximum number of arguments to 16.
This fixes issue where certain operations, combined with table
configured args, exceed 10 arguments.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: 48debafe ("dm: add writecache target")
Cc: stable@vger.kernel.org # v4.18+
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>