FS-Cache: Provide nop fscache_stat_d() if CONFIG_FSCACHE_STATS=n

Provide nop fscache_stat_d() macro if CONFIG_FSCACHE_STATS=n lest errors like
the following occur:

	fs/fscache/cache.c: In function 'fscache_withdraw_cache':
	fs/fscache/cache.c:386: error: implicit declaration of function 'fscache_stat_d'
	fs/fscache/cache.c:386: error: 'fscache_n_cop_sync_cache' undeclared (first use in this function)
	fs/fscache/cache.c:386: error: (Each undeclared identifier is reported only once
	fs/fscache/cache.c:386: error: for each function it appears in.)
	fs/fscache/cache.c:392: error: 'fscache_n_cop_dissociate_pages' undeclared (first use in this function)
Signed-off-by: David Howells <dhowells@redhat.com>

fs/fscache/internal.h

@@ -259,6 +259,7 @@ extern const struct file_operations fscache_stats_fops;
 
 #define __fscache_stat(stat) (NULL)
 #define fscache_stat(stat) do {} while (0)
+#define fscache_stat_d(stat) do {} while (0)
 #endif
 
 /*