dm vdo indexer: don't read request structure after enqueuing

The function get_volume_page_protected may place a request on
a queue for another thread to process asynchronously. When this
happens, the volume should not read the request from the original
thread. This can not currently cause problems, due to the way
request processing is handled, but it is not safe in general.

Reviewed-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>