Use kasprintf instead of kmalloc()-strcpy()-strcat().

Cc: Anton Vorontsov <cbou@mail.ru>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Anton Vorontsov <cbou@mail.ru>