vboxsf: Convert to writepages

If we add a migrate_folio operation, we can convert the writepage
operation to writepages.  Further, this lets us optimise by using
the same write handle for multiple folios.  The large folio support here
is illusory; we would need to kmap each page in turn for proper support.
But we do remove a few hidden calls to compound_head().

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250402150005.2309458-3-willy@infradead.org


Signed-off-by: Christian Brauner <brauner@kernel.org>