NFSv4.1: Allow parallel LOCK/LOCKU calls

Note, however, that we still serialise on the open stateid if the lock
stateid is unconfirmed. Hopefully that will not prove too much of a
burden for first time locks; it should leave the ability to parallelise
OPENs unchanged, since they no longer call the serialisation primitives.

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