nvmet-fc: check successful reference in nvmet_fc_find_target_assoc

When searching for an association based on an association id, when there
is a match, the code takes a reference. However, it is not validating
that the reference taking was successful.

Check the status of the reference. If unsuccessful, the device is being
deleted and should be ignored.

Signed-off-by: James Smart <jsmart2021@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>