af_unix: Refactor unix_read_skb()

Similar to udp_read_skb(), delete the unnecessary while loop in
unix_read_skb() for readability.  Since recv_actor() cannot return a
value greater than skb->len (see sk_psock_verdict_recv()), remove the
redundant check.

Suggested-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Link: https://lore.kernel.org/r/7009141683ad6cd3785daced3e4a80ba0eb773b5.1663909008.git.peilin.ye@bytedance.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>