pnfs: nfs4_proc_layoutget returns void

since the only user of nfs4_proc_layoutget is send_layoutget, which
ignores its return value, there is no reason to return any value.

Signed-off-by: Idan Kedar <idank@tonian.com>
Signed-off-by: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>