NFS: nfs_igrab_and_active must first reference the superblock

commit 896567ee7f17a8a736cda8a28cc987228410a2ac upstream.

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operations that are no longer valid or accessible.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: ea7c38fe ("NFSv4: Ensure we reference the inode for return-on-close in delegreturn")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>