s390/uv: Always return 0 from s390_wiggle_split_folio() if successful

s390/uv: Always return 0 from s390_wiggle_split_folio() if successful

Let's consistently return 0 if the operation was successful, and just
detect ourselves whether splitting is required -- folio_test_large() is
a cheap operation.

Update the documentation.

Should we simply always return -EAGAIN instead of 0, so we don't have
to handle it in the caller? Not sure, staring at the documentation, this
way looks a bit cleaner.

Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250516123946.1648026-3-david@redhat.com
Message-ID: <20250516123946.1648026-3-david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>