NFSv4.0: Fix the wake up of the next waiter in nfs_release_seqid()

There is no need to wake up another waiter on the seqid list unless the
seqid being removed is at the head of the list, and so is relinquishing
control of the sequence counter to the next entry.

Reviewed-by: Yang Erkun <yangerkun@huawei.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>