NOMMU: is_vmalloc_addr() won't compile if !MMU

Make is_vmalloc_addr() contingent on CONFIG_MMU=y, as it won't compile
in !MMU mode.

[ Bug introduced in commit 9e2779fa

:
  "is_vmalloc_addr(): Check if an address is within the vmalloc
  boundaries" ].

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