NFS: Fix a race in nfs_sync_inode()

Kudos to Neil Brown for spotting the problem:

"in nfs_sync_inode, there is effectively the sequence:

   nfs_wait_on_requests
   nfs_flush_inode
   nfs_commit_inode

 This seems a bit racy to me as if the only requests are on the
 ->commit list, and nfs_commit_inode is called separately after
 nfs_wait_on_requests completes, and before nfs_commit_inode start
 (say: by nfs_write_inode) then none of these function will return
 >0, yet there will be some pending request that aren't waited for."

The solution is to search for requests to wait upon, search for dirty
requests, and search for uncommitted requests while holding the
nfsi->req_lock

The patch also cleans up nfs_sync_inode(), getting rid of the redundant
FLUSH_WAIT flag. It turns out that we were always setting it.

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