UPSTREAM: 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.

Bug: 254441685
Fixes: eca4873ee1b6 ("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>
(cherry picked from commit 89ff6005039a878afac87889fee748fa3f957c3a)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I5704830b6c96f6696dba7333eda6309490a2eb34
Signed-off-by: improbir <ghosh_probir@yahoo.com>
Signed-off-by: ij-israfil <israfilkhan494gmail.com>