LU-14768 e2fsck: merge casefolded dir lists after thread finish

LU-14768 e2fsck: merge casefolded dir lists after thread finish

This is missed and should be done after threads finish.

Original commit:
18538b27 ("LU-8465 e2fsck: merge encrypted dir lists after thread finish")
Upstream is now using e2fsck_struct->casefolded_dirs instead of
encrypted_dirs.

Only for the correctness of pfsck, we won't use casefolded feature on
ldiskfs.

Change-Id: I6f3a28501886c2ed4bec26dc5c5e5a1914fece04
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/44010
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>