NFS: Avoid unnecessary rescanning of the per-server delegation list

If the call to nfs_delegation_grab_inode() fails, we will not have
dropped any locks that require us to rescan the list.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>