fs: add field to superblock to support cleancache

This second patch of eight in this cleancache series adds a field to
the generic superblock to squirrel away a pool identifier that is
dynamically provided by cleancache-enabled filesystems at mount time
to uniquely identify files and pages belonging to this mounted filesystem.

Details and a FAQ can be found in Documentation/vm/cleancache.txt

[v8: trivial merge conflict update]
Signed-off-by: Dan Magenheimer <[e-mail]>
Reviewed-by: Jeremy Fitzhardinge <[e-mail]>
Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Nick Piggin <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Rik Van Riel <[e-mail]>
Cc: Jan Beulich <[e-mail]>
Cc: Chris Mason <[e-mail]>
Cc: Andreas Dilger <[e-mail]>
Cc: Ted Ts'o <[e-mail]>
Cc: Mark Fasheh <[e-mail]>
Cc: Joel Becker <[e-mail]>
Cc: Nitin Gupta <[e-mail]>

include/linux/fs.h

···
 	 */
 	char __rcu *s_options;
 	const struct dentry_operations *s_d_op; /* default d_op for dentries */
+
+	/*
+	 * Saved pool identifier for cleancache (-1 means none)
+	 */
+	int cleancache_poolid;
 };
 
 extern struct timespec current_fs_time(struct super_block *sb);