virtio-net: use the check_mergeable_len helper

Replace the current repeated code to check received length in mergeable
mode with the new check_mergeable_len helper.

Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/20250630144212.48471-4-minhquangbui99@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>