f2fs: fix double free of unicode map

In case of retrying fill_super with skip_recovery,
s_encoding for casefold would not be loaded again even though it's
already been freed because it's not NULL.
Set NULL after free to prevent double freeing when unmount.

Fixes: eca4873e ("f2fs: Use generic casefolding support")
Signed-off-by: Hyeongseok Kim <hyeongseok@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>