Fix missing UFFDIO_WRITEPROTECT ioctl for write protecting

[AKKamath]

As per documentation of userfaultfd (https://man7.org/linux/man-pages/man2/userfaultfd.2.html):

   After the UFFDIO_REGISTER ioctl completed with
   UFFDIO_REGISTER_MODE_WP mode set, the user can write-protect any
   existing memory within the range using the ioctl
   UFFDIO_WRITEPROTECT where uffdio_writeprotect.mode should be set
   to UFFDIO_WRITEPROTECT_MODE_WP.

zIO is currently missing the UFFDIO_WRITEPROTECT call after registering source buffers, causing writes to source buffers not to fault. By adding this call, the writes correctly generate faults.