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.

Test-Parameters: mdtcount=4 mdscount=2   env=SANITY_EXCEPT="17e 27J 103 244a"   clientdistro=ubuntu2404 testlist=sanity
Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I58713bfbf22749d6c0a5e40f710549662248e32f
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55984
Reviewed-by: jsimmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>