blk-mq: fix potential oops on out-of-memory in __blk_mq_alloc_rq_maps()

__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: Robert Elliott <Elliott@hp.com>
Signed-off-by: Jens Axboe <axboe@fb.com>