fs: fat: write to non-cluster-aligned root directory

With the commit below, fat now correctly handles a file read under
a non-cluster-aligned root directory of fat12/16.
Write operation should be fixed in the same manner.

Fixes: commit 9b18358d ("fs: fat: fix reading non-cluster-aligned
       root directory")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Anssi Hannula <anssi.hannula@bitwise.fi>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>