fs/isofs: return f_fsid for statfs(2)

Make isofs return f_fsid info for statfs(2).

Signed-off-by: Coly Li <coly.li@suse.de>
Cc: Jan Kara <jack@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>