cifs: abstract out function to marshal up the iovec array for async writes

cifs: abstract out function to marshal up the iovec array for async writes

We'll need to do something a bit different depending on the caller.
Abstract the code that marshals the page array into an iovec.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>