block/iscsi: take iscsilun->mutex in iscsi_timed_check_events()

block/iscsi: take iscsilun->mutex in iscsi_timed_check_events()

Commit d045c466d9e62b4321fadf586d024d54ddfd8bd4 ("iscsi: do not use
aio_context_acquire/release") introduced iscsilun->mutex but appears to
have overlooked iscsi_timed_check_events() when introducing the mutex.

iscsi_service() and iscsi_set_events() must be called with
iscsilun->mutex held.

iscsi_timed_check_events() is invoked from the AioContext and does not
take the mutex.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Message-Id: <20180203061621[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>