Btrfs: clear pages dirty for io and set them extent mapped

Btrfs: clear pages dirty for io and set them extent mapped

When doing the io_ctl helpers to clean up the free space cache stuff I stopped
using our normal prepare_pages stuff, which means I of course forgot to do
things like set the pages extent mapped, which will cause us all sorts of
wonderful propblems.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>