NFSv4: nfs_inode_evict_delegation() should set NFS_DELEGATION_RETURNING

In particular, the pnfs return-on-close code will check for that flag,
so ensure we set it appropriately.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>