nvme-tcp: fix bogus request completion when failing to send AER

AER is not backed by a real request, hence we should not incorrectly
assume that when failing to send a nvme command, it is a normal request
but rather check if this is an aer and if so complete the aer (similar
to the normal completion path).

Cc: stable@vger.kernel.org
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>