selftests/mm: skip gup_longterm tests on weird filesystems

Some filesystems don't support ftruncate()ing unlinked files.  They return
ENOENT.  In that case, skip the test.

Link: https://lkml.kernel.org/r/20250311-mm-selftests-v4-8-dec210a658f5@google.com


Signed-off-by: Brendan Jackman <jackmanb@google.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>