When the value of nbufs is 1, the buffer is in direct mode, which may cause
confusion. So optimizes current codes to make it easier to maintain and
understand.

Link: https://lore.kernel.org/r/1586779091-51410-3-git-send-email-liweihang@huawei.comSigned-off-by: NXi Wang <wangxi11@huawei.com>
Signed-off-by: NLang Cheng <chenglang@huawei.com>
Signed-off-by: NWeihang Li <liweihang@huawei.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>