selftests/vm: use top_srcdir instead of recomputing relative paths

In various places both in t/t/s/v/Makefile as well as some of the test
sources, we were referring to headers or directories using some fairly
long relative paths.

Since we have a working top_srcdir variable though, which refers to the
root of the kernel tree, we can clean up all of these "up and over"
relative paths, just relying on the single variable instead.

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>