NFS: Fix nfs_post_op_update_inode_force_wcc()

If we believe that the attributes are old (see nfs_refresh_inode()), then
we shouldn't force an update.
Also ensure that we hold the inode->i_lock across attribute checks and the
call to nfs_refresh_inode_locked() to ensure that we don't race with other
attribute updates.

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