fbdev: Fix incorrect page mapping clearance at fb_deferred_io_release()

The recent fix for the deferred I/O by the commit
  3efc61d95259 ("fbdev: Fix invalid page access after closing deferred I/O devices")
caused a regression when the same fb device is opened/closed while
it's being used.  It resulted in a frozen screen even if something
is redrawn there after the close.  The breakage is because the patch
was made under a wrong assumption of a single open; in the current
code, fb_deferred_io_release() cleans up the page mapping of the
pageref list and it calls cancel_delayed_work_sync() unconditionally,
where both are no correct behavior for multiple opens.

This patch adds a refcount for the opens of the device, and applies
the cleanup only when all files get closed.

As both fb_deferred_io_open() and _close() are called always in the
fb_info lock (mutex), it's safe to use the normal int for the
refcounting.

Also, a useless BUG_ON() is dropped.

Fixes: 3efc61d95259 ("fbdev: Fix invalid page access after closing deferred I/O devices")
Cc: <[email protected]>
Signed-off-by: Takashi Iwai <[email protected]>
Reviewed-by: Patrik Jakobsson <[email protected]>
Signed-off-by: Thomas Zimmermann <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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