hostfs: Convert to writepages

If we add a migrate_folio operation, we can convert the writepage
operation to writepages.  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://patch.msgid.link/20241220051500.1919389-1-willy@infradead.org


Signed-off-by: Johannes Berg <johannes.berg@intel.com>