NFSv4: Don't trigger uneccessary scans for return-on-close delegations

The amount of looping through the list of delegations is occasionally
leading to soft lockups. Avoid at least some loops by not requiring the
NFSv4 state manager to scan for delegations that are marked for
return-on-close. Instead, either mark them for immediate return (if
possible) or else leave it up to nfs4_inode_return_delegation_on_close()
to return them once the file is closed by the application.

Fixes: b757144f ("NFSv4: Be less aggressive about returning delegations for open files")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>