netfs: Provide a writepages implementation

Provide an implementation of writepages for network filesystems to delegate
to.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org

include/linux/netfs.h

@@ -397,6 +397,8 @@ int netfs_read_folio(struct file *, struct folio *);
 int netfs_write_begin(struct netfs_inode *, struct file *,
 		      struct address_space *, loff_t pos, unsigned int len,
 		      struct folio **, void **fsdata);
+int netfs_writepages(struct address_space *mapping,
+		     struct writeback_control *wbc);
 bool netfs_dirty_folio(struct address_space *mapping, struct folio *folio);
 int netfs_unpin_writeback(struct inode *inode, struct writeback_control *wbc);
 void netfs_clear_inode_writeback(struct inode *inode, const void *aux);