NFS: Fix inode fileid checks in attribute revalidation code

We want to throw out the attrbute if it refers to the mounted on fileid,
and not the real fileid. However we do not want to block cache consistency
updates from NFSv4 writes.

Reported-by: Murphy Zhou <jencce.kernel@gmail.com>
Fixes: 7e10cc25

 ("NFS: Don't refresh attributes with mounted-on-file...")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>