mirror: Fix access of uninitialised fields during start

mirror: Fix access of uninitialised fields during start

bdrv_mirror_top_pwritev() accesses the job object when active mirroring
is enabled. It disables this code during early initialisation while
s->job isn't set yet.

However, s->job is still set way too early when the job object isn't
fully initialised. For example, &s->ops_in_flight isn't initialised yet
and the in_flight bitmap doesn't exist yet. This causes crashes when a
write request comes in too early.

Move the assignment of s->job to when the mirror job is actually fully
initialised to make sure that the mirror_top driver doesn't access it
too early.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230203152202.49054-3-kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>