Modify the handling of getattr for the DS attributes so that instead
of failing for the case where the pnfsd.dsattr extended attribute does not
exist, a Getattr RPC against the DS is attempted to (re)create this
extended attribute.

This allows a sysadmin to "rmextattr system pnfsd.dsattr <filename>" on
the MDS exported file system file, if these attributes somehow get messed up.