do_pollfd(): convert to CLASS(fd)

lift setting ->revents into the caller, so that failure exits (including
the early one) would be plain returns.

We need the scope of our struct fd to end before the store to ->revents,
since that's shared with the failure exits prior to the point where we
can do fdget().

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>