NFS: nfs_delegation_find_inode_server must first reference the superblock

Before referencing the inode, we must ensure that the superblock can be
referenced. Otherwise, we can end up with iput() calling superblock
operations that are no longer valid or accessible.

Fixes: e39d8a18 ("NFSv4: Fix an Oops during delegation callbacks")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>