fbcon: add lifetime refcount to opened frame buffers

This just adds the refcount and the new registration lock logic.  It
does not (for example) actually change the read/write/ioctl routines to
actually use the frame buffer that was opened: those function still end
up alway susing whatever the current frame buffer is at the time of the
call.

Without this, if something holds the frame buffer open over a
framebuffer switch, the close() operation after the switch will access a
fb_info that has been free'd by the unregistering of the old frame
buffer.

(The read/write/ioctl operations will normally not cause problems,
because they will - illogically - pick up the new fbcon instead.  But a
switch that happens just as one of those is going on might see problems
too, the window is just much smaller: one individual op rather than the
whole open-close sequence.)

This use-after-free is apparently fairly easily triggered by the Ubuntu
11.04 boot sequence.

Acked-by: Tim Gardner <tim.gardner@canonical.com>
Tested-by: Daniel J Blueman <daniel.blueman@gmail.com>
Tested-by: Anca Emanuel <anca.emanuel@gmail.com>
Cc: Bruno Prémont <bonbons@linux-vserver.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Andy Whitcroft <andy.whitcroft@canonical.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>