NFS: simplify the detached delegation check in update_open_stateid

NFS: simplify the detached delegation check in update_open_stateid

When nfs_detach_delegation_locked detaches a delegation from an inode,
it clears both nfsi->delegation and delegation->inode.  Use the later
in update_open_stateid to check for a detached inode, as that avoids
an extra local variable, and removes the need for a RCU derefernence
as we already hold the lock in the delegation.  This prepares for
removing the surrounding RCU critical section.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>