nommu: stub expand_stack() for nommu case

Be consistent with VM mmap, implement expand_stack().  We can't actually do
anything other than return an error in the no MMU case though.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>