NFS: Fix an Oops in nfs_d_automount()

When mounting from a NFSv4 referral, path->dentry can end up being a
negative dentry, so derive the struct nfs_server from the dentry
itself instead.

Fixes: 2b0143b5

 ("VFS: normal filesystems (and lustre): d_inode() annotations")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>