fbdev/defio: Early-out if page is already enlisted

fbdev/defio: Early-out if page is already enlisted

[ Upstream commit 105a940416fc622406653b6fe54732897642dfbc ]

Return early if a page is already in the list of dirty pages for
deferred I/O. This can be detected if the page's list head is not
empty. Keep the list head initialized while the page is not enlisted
to make this work reliably.

v2:
	* update comment and fix spelling (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220211094640.21632-2-tzimmermann@suse.de
Stable-dep-of: 33cd6ea9c067 ("fbdev: flush deferred IO before closing")
Signed-off-by: Sasha Levin <sashal@kernel.org>