NFS: Fix missing unlock in nfs_unlink()

Add the missing unlock before goto.

Fixes: 3c59366c ("NFS: don't unhash dentry during unlink/rename")
Signed-off-by: Sun Ke <sunke32@huawei.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>