The PagemapEntry and struct iovec are quite interchangeable.
PagemapEntry can be used directly in the few places that converted it to
struct iovec.
The only place that used iovec2pagemap can be switched to open-coded
assignments without really hurting code readability.

travis-ci: success for pagemap: minor cleanups (rev2)
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>