->poll() instances shouldn't be indefinitely blocking

[Al Viro]

Take a look at this:
static unsigned int gsc_m2m_poll(struct file *file,
                                        struct poll_table_struct *wait)
{
        struct gsc_ctx *ctx = fh_to_ctx(file->private_data);
        struct gsc_dev *gsc = ctx->gsc_dev;
        int ret;

        if (mutex_lock_interruptible(&gsc->lock))
                return -ERESTARTSYS;

        ret = v4l2_m2m_poll(file, ctx->m2m_ctx, wait);
        mutex_unlock(&gsc->lock);

        return ret;
}

a) ->poll() should not return -E...; callers expect just a bitmap of
POLL... values.

b) sure, it's nice that if this thing hangs, we'll be able to kill it.
However, if one's ->poll() can hang indefinitely, it means bad things
for poll(2), select(2), etc. semantics.  What the hell had been intended
there?

c) a bunch of v4l2_m2m_poll() callers are also taking some kind of
mutex; AFAICS, all of those appear bogus (the rest of them do not
play wiht ERESTARTSYS, just plain mutex_lock() for those).

What's going on there?
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