fb_defio: Remove custom address_space_operations

fb_defio: Remove custom address_space_operations

There's no need to give the page an address_space.  Leaving the
page->mapping as NULL will cause the VM to handle set_page_dirty()
the same way that it's handled now, and that was the only reason to
set the address_space in the first place.

Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]