Merge branch 'af_unix-fix-so_peek_off-bug-in-unix_stream_read_generic'

Kuniyuki Iwashima says:

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af_unix: Fix SO_PEEK_OFF bug in unix_stream_read_generic().

Miao Wang reported a bug of SO_PEEK_OFF on AF_UNIX SOCK_STREAM socket.

Patch 1 fixes the bug and Patch 2 adds a new selftest to cover the case.
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Link: https://patch.msgid.link/20251117174740.3684604-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>