io: Make queue_io_read() safe for reentry

io: Make queue_io_read() safe for reentry

queue_io_read() may be reentered via the queue callback, recoverd is
particularly guilty of this.

queue_io_read() is not safe for reentry if more than one packet is
received and partial chunks follow - data read off the pipe on re-entry
is assumed to be the start-of-packet four byte length. This leads to a
wrongly aligned stream and the notorious "Invalid packet of length 0"
errors.

This change fixes queue_io_read() to be safe under reentry, only a
single packet is processed per call.

https://bugzilla.samba.org/show_bug.cgi?id=8319