vcs: poll(): cope with a deallocated vt

When VT_DISALLOCATE is used on a vt, user space waiting with poll() on
the corresponding /dev/vcs device is not awakened. This is now fixed by
returning POLLHUP|POLLERR to user space.

Also, in the normal screen update case, we don't set POLLERR anymore as
POLLPRI alone is a much more logical response in a non-error situation,
saving some confusion on the user space side. The only known user app
making use of poll() on /dev/vcs* is BRLTTY which is known to cope with
that change already, so the risk of breakage is pretty much nonexistent.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>