NFSv4: Fix a potential CLOSE race

 Once the state_owner and lock_owner semaphores get removed, it will be
 possible for other OPEN requests to reopen the same file if they have
 lower sequence ids than our CLOSE call.
 This patch ensures that we recheck the file state once
 nfs_wait_on_sequence() has completed waiting.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>