vboxsf: Use a folio in vboxsf_write_end()

Because we have to kmap() the page before calling vboxsf_write(), we
can't entirely remove the use of struct page.  But we can eliminate some
uses of old APIs and remove some unnecessary calls to compound_head().

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>