af_unix: Read with MSG_PEEK loops if the first unread byte is OOB

Read with MSG_PEEK flag loops if the first byte to read is an OOB byte.
commit 22dd70eb ("af_unix: Don't peek OOB data without MSG_OOB.")
addresses the loop issue but does not address the issue that no data
beyond OOB byte can be read.

>>> from socket import *
>>> c1, c2 = socketpair(AF_UNIX, SOCK_STREAM)
>>> c1.send(b'a', MSG_OOB)
1
>>> c1.send(b'b')
1
>>> c2.recv(1, MSG_PEEK | MSG_DONTWAIT)
b'b'

>>> from socket import *
>>> c1, c2 = socketpair(AF_UNIX, SOCK_STREAM)
>>> c2.setsockopt(SOL_SOCKET, SO_OOBINLINE, 1)
>>> c1.send(b'a', MSG_OOB)
1
>>> c1.send(b'b')
1
>>> c2.recv(1, MSG_PEEK | MSG_DONTWAIT)
b'a'
>>> c2.recv(1, MSG_PEEK | MSG_DONTWAIT)
b'a'
>>> c2.recv(1, MSG_DONTWAIT)
b'a'
>>> c2.recv(1, MSG_PEEK | MSG_DONTWAIT)
b'b'
>>>

Fixes: 314001f0 ("af_unix: Add OOB support")
Signed-off-by: Rao Shoaib <Rao.Shoaib@oracle.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240611084639.2248934-1-Rao.Shoaib@oracle.com


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