migration/ram: Fix error handling in ram_write_tracking_start()

migration/ram: Fix error handling in ram_write_tracking_start()

If something goes wrong during uffd_change_protection(), we would miss
to unregister uffd-wp and not release our reference. Fix it by
performing the uffd_change_protection(true) last.

Note that a uffd_change_protection(false) on the recovery path without a
prior uffd_change_protection(false) is fine.

Fixes: 278e2f551a09 ("migration: support UFFD write fault processing in ram_save_iterate()")
Cc: qemu-stable@nongnu.org
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
(cherry picked from commit 72ef3a370836aa07261ad7aaeea27ed5cbcee342)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>