selftests/mm/vm_util: robust write_file()

selftests/mm/vm_util: robust write_file()

Add three more checks for buflen and numwritten.  The buflen should be at
least two, that means at least one char and the null-end.  The error case
check is added by checking numwriten < 0 instead of numwritten < 1.  And
the truncate case is checked.  The test will exit if any of these
conditions aren't met.

Additionally, add more print information when a write failure occurs or a
truncated write happens, providing clearer diagnostics.

Link: https://lore.kernel.org/20260402014543.1671131-5-chuhu@redhat.com
Signed-off-by: Chunyu Hu <chuhu@redhat.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>