NFS: Add a super_block backpointer to the nfs_server struct

NFS_SB() returns the pointer to an nfs_server struct, given a
pointer to a super_block.  But we have no way to go back the other
way.

Add a super_block backpointer field so that, given an nfs_server
struct, it is easy to get to the filesystem's root dentry.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/super.c

@@ -2534,6 +2534,7 @@ struct dentry *nfs_fs_mount_common(struct nfs_server *server,
 			mntroot = ERR_PTR(error);
 			goto error_splat_bdi;
 		}
+		server->super = s;
 	}
 
 	if (!s->s_root) {

include/linux/nfs_fs_sb.h

@@ -148,6 +148,7 @@ struct nfs_server {
 	__u64			maxfilesize;	/* maximum file size */
 	struct timespec		time_delta;	/* smallest time granularity */
 	unsigned long		mount_time;	/* when this fs was mounted */
+	struct super_block	*super;		/* VFS super block */
 	dev_t			s_dev;		/* superblock dev numbers */
 
 #ifdef CONFIG_NFS_FSCACHE