LU-17453 llite: use dget_parent to access dentry.d_parent

LU-17453 llite: use dget_parent to access dentry.d_parent

Use dget_parent() to aquire the d_parent member of a dentry
to ensure dentry is valid while it is accessed.

HPE-bug-id: LUS-11889
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: Icb0a25ece5a3a3d50da076708fcd631176652a1b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53757
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>