NFSv4.1: fix handling NFS4ERR_DELAY when testing for session trunking

Currently when client sends an EXCHANGE_ID for a possible trunked
connection, for any error that happened, the trunk will be thrown
out. However, an NFS4ERR_DELAY is a transient error that should be
retried instead.

Fixes: e818bd08 ("NFSv4.1 remove xprt from xprt_switch if session trunking test fails")
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>