NFSv4.x: Drop the slot if nfs4_delegreturn_prepare waits for layoutreturn

If nfs4_delegreturn_prepare needs to wait for a layoutreturn to complete
then make sure we drop the sequence slot if we hold it.

Fixes: 1c5bd76d

 ("pNFS: Enable layoutreturn operation for return-on-close")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>