LU-14629 sec: do not block rename of topmost encrypted dir

LU-14629 sec: do not block rename of topmost encrypted dir

We intentionally forbid file and directory rename from encrypted to
unencrypted directory. But we must not block rename of the topmost
encrypted directory.

Fixes: 1158386ac9 ("LU-14629 sec: forbid file rename from enc to unencrypted dir")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I480a24b2b0327e1d9104f216da54720e4f351636
Reviewed-on: https://review.whamcloud.com/45054
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>