ceph: clean up unsafe d_parent accesses in build_dentry_path

ceph: clean up unsafe d_parent accesses in build_dentry_path

BugLink: http://bugs.launchpad.net/bugs/1729105
[ Upstream commit c6b0b656ca24ede6657abb4a2cd910fa9c1879ba ]

While we hold a reference to the dentry when build_dentry_path is
called, we could end up racing with a rename that changes d_parent.
Handle that situation correctly, by using the rcu_read_lock to
ensure that the parent dentry and inode stick around long enough
to safely check ceph_snap and ceph_ino.

Link: http://tracker.ceph.com/issues/18148
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>