Reduce the number of getattr/lookup calls in nfs_lookup_revalidate()
by making the latter rely on the ordinary attribute cache, and moving
the close-to-open data consistency checking into nfs_open().

This does mean that we can end up calling GETATTR twice: once in
nfs_lookup_revalidate() then immediately after in nfs_open(), however
it also means that sys_stat() and friends now use cached attributes.