__blk_mq_alloc_rq_maps() can be invoked multiple times, if we scale
back the queue depth if we are low on memory. So don't clear
set->tags when we fail, this is handled directly in
the parent function, blk_mq_alloc_tag_set().
Reported-by: NRobert Elliott  <Elliott@hp.com>
Signed-off-by: NJens Axboe <axboe@fb.com>