sctp: do not pr_err for the duplicated node in transport rhlist

The pr_err in sctp_hash_transport was supposed to report a sctp bug
for using rhashtable/rhlist.

The err '-EEXIST' introduced in Commit cd2b7087 ("sctp: check
duplicate node before inserting a new transport") doesn't belong
to that case.

So just return -EEXIST back without pr_err any kmsg.

Fixes: cd2b7087 ("sctp: check duplicate node before inserting a new transport")
Reported-by: Wei Chen <weichen@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>