af_unix: fix incorrect revert of 'lock_interruptible' in stream receive code

As reported by Sultan Qasim, commit 3822b5c ("af_unix: Revert
'lock_interruptible' in stream receive code") was accidently applied
at the wrong place in the backport that appeared in 3.10.95, it
affected unix_dgram_recvmsg() instead of unix_stream_recvmsg() due
to now similar code sections there. The dgram part needs to remain
but the stream part needs to be removed.

Reported-By: Sultan Qasim <sultanqasim@gmail.com>
Fixes: 3a57e78 (3.10.95)
Signed-off-by: Willy Tarreau <w@1wt.eu>