netfs: Implement buffered write API

netfs: Implement buffered write API

Institute a netfs write helper, netfs_file_write_iter(), to be pointed at
by the network filesystem ->write_iter() call.  Make it handled buffered
writes by calling the previously defined netfs_perform_write() to copy the
source data into the pagecache.

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