tools/testing/selftests: fix gup_longterm for unknown fs

Commit 66bce7afbaca ("selftests/mm: fix test result reporting in
gup_longterm") introduced a small bug causing unknown filesystems to
always result in a test failure.

This is because do_test() was updated to use a common reporting path, but
this case appears to have been missed.

This is problematic for e.g. virtme-ng which uses an overlayfs file
system, causing gup_longterm to appear to fail each time due to a test
count mismatch:

# Planned tests != run tests (50 != 46)
# Totals: pass:24 fail:0 xfail:0 xpass:0 skip:22 error:0

The fix is to simply change the return into a break.

Link: https://lkml.kernel.org/r/20260106154547.214907-1-lorenzo.stoakes@oracle.com
Fixes: 66bce7afbaca ("selftests/mm: fix test result reporting in gup_longterm")
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>