fbdev: Refactor implementation of page_mkwrite

Refactor the page-write handler for deferred I/O. Drivers use the
function to let fbdev track written pages of mmap'ed framebuffer
memory.

v3:
	* keep locking within track-pages function for readability (Sam)
v2:
	* don't export the helper until we have an external caller

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220429100834.18898-4-tzimmermann@suse.de