selftests/mm: fix test result reporting in gup_longterm

The kselftest framework uses the string logged when a test result is
reported as the unique identifier for a test, using it to track test
results between runs.  The gup_longterm test fails to follow this pattern,
it runs a single test function repeatedly with various parameters but each
result report is a string logging an error message which is fixed between
runs.

Since the code already logs each test uniquely before it starts refactor
to also print this to a buffer, then use that name as the test result. 
This isn't especially pretty but is relatively straightforward and is a
great help to tooling.

Link: https://lkml.kernel.org/r/20250527-selftests-mm-cow-dedupe-v2-4-ff198df8e38e@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>