NFS: Make read() return an ESTALE if the file has been deleted

Currently, a read() request will return EIO even if the file has been
deleted on the server, simply because that is what the VM will return
if the call to readpage() fails to update the page.

Ensure that readpage() marks the inode as stale if it receives an ESTALE.
Then return that error to userland.

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