vdpa: Add documentation for vdpa_alloc_device() macro

mainline inclusion
from mainline-v5.14-rc7
commit c8d182bd
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c8d182bd387a09a8b95303c8086238e8bf61fcfc



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The return value of vdpa_alloc_device() macro is not very
clear, so that most of callers did the wrong check. Let's
add some comments to better document it.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20210715080026.242-4-xieyongji@bytedance.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Pengyuan Zhao <zhaopengyuan@hisilicon.com>