NFSv4: Refactor _nfs4_proc_exchange_id()

Tease apart the functionality in nfs4_exchange_id_done() so that
it is easier to debug exchange id vs trunking issues by moving
all the processing out of nfs4_exchange_id_done() and into the
callers.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>