fbdev/deferred-io: Clean up pageref on lastclose

fbdev/deferred-io: Clean up pageref on lastclose

Clean up the pageref state as part of the lastclose helper. This
only requires to clear the page's mapping field. The pageref and
page can stay in place for the next opened instance of the frame-
buffer file.

With the change in the clean-up logic, there's no further need
to look up pages during the lastclose cleanup. The code instead
uses the existing pagerefs in its look-up table. It also avoids
using smem_len, which some driver might not set correctly.

v2:
- fix typos in commit message (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-4-tzimmermann@suse.de