fs, nfs: convert nfs_client.cl_count from atomic_t to refcount_t

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable nfs_client.cl_count is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

include/linux/nfs_fs_sb.h

@@ -9,6 +9,7 @@
 #include <linux/sunrpc/xprt.h>
 
 #include <linux/atomic.h>
+#include <linux/refcount.h>
 
 struct nfs4_session;
 struct nfs_iostats;
@@ -24,7 +25,7 @@ struct nfs41_impl_id;
  * The nfs_client identifies our client state to the server.
  */
 struct nfs_client {
-	atomic_t		cl_count;
+	refcount_t		cl_count;
 	atomic_t		cl_mds_count;
 	int			cl_cons_state;	/* current construction state (-ve: init error) */
 #define NFS_CS_READY		0		/* ready to be used */