net: do not write to msg_get_inq in callee

In the Linux kernel, the following vulnerability has been resolved:

net: do not write to msg_get_inq in callee

NULL pointer dereference fix.

msg_get_inq is an input field from caller to callee. Don't set it in
the callee, as the caller may not clear it on struct reuse.

This is a kernel-internal variant of msghdr only, and the only user
does reinitialize the field. So this is not critical for that reason.
But it is more robust to avoid the write, and slightly simpler code.
And it fixes a bug, see below.

Callers set msg_get_inq to request the input queue length to be
returned in msg_inq. This is equivalent to but independent from the
SO_INQ request to return that same info as a cmsg (tp->recvmsg_inq).
To reduce branching in the hot path the second also sets the msg_inq.
That is WAI.

This is a fix to commit 4d1442979e4a ("af_unix: don't post cmsg for
SO_INQ unless explicitly asked for"), which fixed the inverse.

Also avoid NULL pointer dereference in unix_stream_read_generic if
state->msg is NULL and msg->msg_get_inq is written. A NULL state->msg
can happen when splicing as of commit 2b514574f7e8 ("net: af_unix:
implement splice for stream af_unix sockets").

Also collapse two branches using a bitwise or.