NFS: Fix RCU dereference of cl_xprt in nfs_compare_super_address

NFS: Fix RCU dereference of cl_xprt in nfs_compare_super_address

The cl_xprt pointer in struct rpc_clnt is marked as __rcu. Accessing
it directly in nfs_compare_super_address() is unsafe and triggers
Sparse warnings.

Fix this by using rcu_dereference() within an RCU read-side critical
section to retrieve the transport pointer. This addresses the sparse
warning and ensures atomic access to the pointer, as the transport
can be updated via transport switching even while the superblock
remains active under sb_lock.

Fixes: 7e3fcf61abde ("nfs: don't share mounts between network namespaces")
Signed-off-by: Sean Chang <seanwascoding@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>