vfs: Export file system uuid via /proc/<pid>/mountinfo

We add a per superblock uuid field. File systems should
update the uuid in the fill_super callback

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -1408,6 +1408,7 @@ struct super_block {
 	wait_queue_head_t	s_wait_unfrozen;
 
 	char s_id[32];				/* Informational name */
+	u8 s_uuid[16];				/* UUID */
 
 	void 			*s_fs_info;	/* Filesystem private info */
 	fmode_t			s_mode;