LU-18085 llite: use RCU to protect the dentry_data

LU-18085 llite: use RCU to protect the dentry_data

The upstream has changed the rule of dentry kill since
v6.7-rc1-20-g1c18edd1b7a0. The d_release callback will
be invoked before the dentry was removed from children
list. This means the changes of d_fsdata could be seen
for others. We have already used call_rcu to handle the
release. So just apply RCU in read side to ensure access
safety.

Lustre-change: https://review.whamcloud.com/55984
Lustre-commit: 983999bda71115595df48d614ca1aaf9b746c75f

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I58713bfbf22749d6c0a5e40f710549662248e32f
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57007
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>