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

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

BugLink: https://bugs.launchpad.net/bugs/1861085
commit 5326de9e94bedcf7366e7e7625d4deb8c1f1ca8a upstream.

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: 1c5bd76d17cc ("pNFS: Enable layoutreturn operation for return-on-close")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>