NFS: Do not report writeback errors in nfs_getattr()

commit d19e0183 upstream.

The result of the writeback, whether it is an ENOSPC or an EIO, or
anything else, does not inhibit the NFS client from reporting the
correct file timestamps.

Fixes: 79566ef0

 ("NFS: Getattr doesn't require data sync semantics")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>