VFS: trivial: fix comment on s_maxbytes value warning check

I originally intended to remove this warning in 2.6.34, but it's not in
a high performance codepath and might help us to catch bugs later. Let's
keep it, but fix the comment to allay confusion about its removal.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>