drbd: Fixed w_restart_disk_io() to handle non active AL-extents

Since we now apply the AL in user space onto the bitmap, the AL
is not active for the requests we want to reply.

For that a al_write_transaction() that might be called from
worker context became necessary.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>