nvme_fc: revise comment on teardown

nvme_fc: revise comment on teardown

Per the recommendation by Sagi on:
http://lists.infradead.org/pipermail/linux-nvme/2017-April/009261.html

An extra reference was pointed out.  There's no issue with the
references, but rather a literal interpretation of what the comment
is saying.

Reword the comment to avoid confusion.

Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>