mm: Update is_vmalloc_addr to account for vmalloc savings

is_vmalloc_addr current assumes that all vmalloc addresses
exist between VMALLOC_START and VMALLOC_END. This may not be
the case when interleaving vmalloc and lowmem. Update the
is_vmalloc_addr to properly check for this.

Change-Id: I5def3d6ae1a4de59ea36f095b8c73649a37b1f36
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>