Declare get_vmalloc_info_fn only when needed

A couple of definitions were outside a preprocessor conditional
statement when they should have been inside it. This caused build
failures for some people.

Closes zfsonlinux/spl#291

Reported-by: Aaron Fineman <abyxcos@mnetic.ch>
Signed-off-by: Richard Yao <ryao@gentoo.org>