net/http: fix hijack hang at abortPendingRead

[jing-rui]

When concurrent many http requests which need hijack the connenction,
hijack may hang at abortPendingRead() on cr.cond.Wait().

The read can be from readRequest() from conn.serve() or
w.conn.r.startBackgroundRead(). especially in startBackgroundRead()
which always set the deadline time.Time{}.

This problem seems easy reproduce on arm.

Signed-off-by: jingrui [email protected]

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