selftests/vm: fix inability to build any vm tests

When we stopped using KSFT_KHDR_INSTALL, a side effect is we also
changed the value of `top_srcdir`. This can be seen by looking at the
code removed by commit 49de12ba
("selftests: drop KSFT_KHDR_INSTALL make target").

(Note though that this commit didn't break this, technically the one
before it did since that's the one that stopped KSFT_KHDR_INSTALL from
being used, even though the code was still there.)

Previously lib.mk reconfigured `top_srcdir` when KSFT_KHDR_INSTALL was
being used. Now, that's no longer the case.

As a result, the path to gup_test.h in vm/Makefile was wrong, and
since it's a dependency of all of the vm binaries none of them could
be built. Instead, we'd get an "error" like:

    make[1]: *** No rule to make target
        '/[...]/tools/testing/selftests/vm/compaction_test', needed by
	'all'.  Stop.

So, modify lib.mk so it once again sets top_srcdir to the root of the
kernel tree.

Fixes: f2745dc0

 ("selftests: stop using KSFT_KHDR_INSTALL")
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>