NFS: nfs_mark_for_revalidate should always set NFS_INO_REVAL_PAGECACHE

I'm not aware of any existing bugs around this, but the expectation is
that nfs_mark_for_revalidate() should always force a revalidation of
the cached metadata.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>