nfs4: verify seqid in OPEN_CONFIRM as per RFC

According to the RFC 3530 (14.2.18):

    The sequence id passed to the OPEN_CONFIRM must be 1 (one) greater
    than the seqid passed to the OPEN operation from which the open_confirm
    value was obtained.

But we were verifying opopen_confirm.seqid against stateid.seqid.  This
compares it to opopen.seqid + 1 instead.
Merge pull request #2 from nla/open-confirm-seqid

nfs4: verify seqid in OPEN_CONFIRM as per RFC