Add checks for a Layout plus Open/Write_access or Write_delegation to the
nfsrv_proxyds() function for the Getattr case. Do the attribute updates for
Close and DelegReturn for the write cases.
This should make it perform better for clients such as Linux, where the
value of vfs.nfsd.pnfsgetdsattr needs to be set non-zero to keep the file's
attributes (mainly size) up to date.