fs: fat: extend get_fs_info() for write use

get_fs_info() was introduced in major re-work of read operation by Rob.
We want to reuse this function in write operation by extending it with
additional members in fsdata structure.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>