selftests: make use of GUP_TEST_FILE macro

Commit 17de1e55 ("selftests: clarify common error when running
gup_test") had most of its hunks dropped due to a conflict with another
patch accepted into Linux around the same time that implemented the same
behavior as a subset of other changes.

However, the remaining hunk defines the GUP_TEST_FILE macro without
making use of it. This patch makes use of the macro in the two relevant
places.

Furthermore, the above mentioned commit's log message erroneously describes
the changes that were dropped from the patch.

This patch corrects the record.

Fixes: 17de1e55 ("selftests: clarify common error when running gup_test")

Signed-off-by: Joel Savitz <jsavitz@redhat.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Nico Pache <npache@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>