nvmet-fc: move RCU read lock to nvmet_fc_assoc_exists

The RCU lock is only needed for the lookup loop and not for
list_ad_tail_rcu call. Thus move it down the call chain into
nvmet_fc_assoc_exists.

While at it also fix the name typo of the function.

Signed-off-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>