NFS: Refactor nfs_lookup_revalidate()

commit 5ceb9d7fdaaf6d8ced6cd7861cf1deb9cd93fa47 upstream.

Refactor the code in nfs_lookup_revalidate() as a stepping stone towards
optimising and fixing nfs4_lookup_revalidate().

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Qian Lu <luqia@amazon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>