fs: clarify "dummy" member in struct inodes_stat_t

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>