Overlayfs: Wrap RCU-mode accesses to ->d_inode on subordinate filesystems

There is a place in overlayfs where it may touch a lower-layer inode during
RCU-mode pathwalk.  In this situation the combination of d_backing_inode() and
ACCESS_ONCE() does not compile, so institute and use a d_backing_inode_rcu()
function that combines these.

Signed-off-by: David Howells <dhowells@redhat.com>