FROMGIT: block/blk-rq-qos: Move a function from a header file into a C file

rq_qos_id_to_name() is only used in blk-mq-debugfs.c so move that function
into in blk-mq-debugfs.c.

Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
BUG: 187357408
Change-Id: If03083a13917bc2f88b6df7151e033a11ab1bc50
(cherry picked from commit f1a7f539c2720906fb10be0af3514b034e1a9fee git://git.kernel.dk/linux-block/

 for-5.14/block)
Signed-off-by: Bart Van Assche <bvanassche@google.com>