LU-18138 mdd: handle corner case for .lustre on readdir

LU-18138 mdd: handle corner case for .lustre on readdir

If '.lustre' entry is dispaced into last directory block and is alone
in it - the entry to returned by mdd_readdir.

Test to illustrate the issus is added.

Fixes: e6b8e3a86b ("LU-5044 mdd: do not return .lustre from readdir")
Signed-off-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Change-Id: Ib4dfe92b66932ede1b1336e55f1af33c1fc6c878
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56021
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>