selftests/mm: ksm-functional-tests: fix partial write handling

selftests/mm: ksm-functional-tests: fix partial write handling

Update write() checks to properly detect and handle partial writes.

Previously, the write() calls used <= 0 to detect failure.  This condition
is never true for partial writes (ret > 0 but ret < len), so partial
writes were silently treated as success.

Fix this by verifying that write() returns the full expected length and
treating any mismatch as failure.

Link: https://lore.kernel.org/20260504081638.683223-1-agarwal.vineet2006@gmail.com
Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>