fs: fat: Fix cache align error message in fatwrite

Use of malloc of do_fat_write() causes cache error on ARM v7 platforms.
Perhaps, the same problem will occur at any other CPUs.
This replaces malloc with memalign to fix cache buffer alignment.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Yoshiyuki Ito <yoshiyuki.ito.ub@renesas.com>
Tested-by: Hector Palacios <hector.palacios@digi.com>