nfs: Update server port after referral or migration

After traversing a referral or recovering from a migration event,
ensure that the server port reported in /proc/mounts is updated
to the correct port setting for the new submount.

Reported-by: Helen Chao <helen.chao@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>