proc: introduce proc_create_single{,_data}

Variants of proc_create{,_data} that directly take a seq_file show
callback and drastically reduces the boilerplate code in the callers.

All trivial callers converted over.

Signed-off-by: Christoph Hellwig <hch@lst.de>

fs/fscache/internal.h

@@ -295,7 +295,7 @@ static inline void fscache_stat_d(atomic_t *stat)
 
 #define __fscache_stat(stat) (stat)
 
-extern const struct file_operations fscache_stats_fops;
+int fscache_stats_show(struct seq_file *m, void *v);
 #else
 
 #define __fscache_stat(stat) (NULL)