NFS: fix PROC_FS=n compile error

fs/built-in.o: In function `nfs_show_stats':inode.c:(.text+0x15481a): undefined reference to `rpc_print_iostats'
net/built-in.o: In function `rpc_destroy_client': undefined reference to `rpc_free_iostats'
net/built-in.o: In function `rpc_clone_client': undefined reference to `rpc_alloc_iostats'
net/built-in.o: In function `rpc_new_client': undefined reference to `rpc_alloc_iostats'
net/built-in.o: In function `xprt_release': undefined reference to `rpc_count_iostats'
make: *** [.tmp_vmlinux1] Error 1

Signed-off-by: Adrian Bunk <[e-mail]>
Cc: Trond Myklebust <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

include/linux/sunrpc/metrics.h

···
 /*
  * EXPORTed functions for managing rpc_iostats structures
  */
+
+#ifdef CONFIG_PROC_FS
+
 struct rpc_iostats *	rpc_alloc_iostats(struct rpc_clnt *);
 void			rpc_count_iostats(struct rpc_task *);
 void			rpc_print_iostats(struct seq_file *, struct rpc_clnt *);
 void			rpc_free_iostats(struct rpc_iostats *);
+
+#else  /*  CONFIG_PROC_FS  */
+
+static inline struct rpc_iostats *rpc_alloc_iostats(struct rpc_clnt *clnt) { return NULL; }
+static inline void rpc_count_iostats(struct rpc_task *task) {}
+static inline void rpc_print_iostats(struct seq_file *seq, struct rpc_clnt *clnt) {}
+static inline void rpc_free_iostats(struct rpc_iostats *stats) {}
+
+#endif  /*  CONFIG_PROC_FS  */
 
 #endif /* _LINUX_SUNRPC_METRICS_H */