mm: make is_vmalloc_addr work properly.

There was a typo in the config guard for CONFIG_ENABLE_VMALLOC_SAVING
which meant that the code was never actually being compiled. As a
result, it was never noticed that the code had major flaws. Fix
the code to actually work as intended.

Change-Id: Ief3c00d16cf54e3b945ffb1bfde6b1fea2fa142e
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>