NFS: Fix use of nfs_attr_use_mounted_on_fileid()

This function call was being optimized out during nfs_fhget(), leading
to situations where we have a valid fileid but still want to use the
mounted_on_fileid.  For example, imagine we have our server configured
like this:

server % df
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1       9.1G  6.5G  1.9G  78% /
/dev/vdb1       487M  2.3M  456M   1% /exports
/dev/vdc1       487M  2.3M  456M   1% /exports/vol1
/dev/vdd1       487M  2.3M  456M   1% /exports/vol2

If our client mounts /exports and tries to do a "chown -R" across the
entire mountpoint, we will get a nasty message warning us about a circular
directory structure.  Running chown with strace tells me that each directory
has the same device and inode number:

newfstatat(AT_FDCWD, "/nfs/", {st_dev=makedev(0, 38), st_ino=2, ...}) = 0
newfstatat(4, "vol1", {st_dev=makedev(0, 38), st_ino=2, ...}) = 0
newfstatat(4, "vol2", {st_dev=makedev(0, 38), st_ino=2, ...}) = 0

With this patch the mounted_on_fileid values are used for st_ino, so the
directory loop warning isn't reported.
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/inode.c

@@ -352,8 +352,9 @@ nfs_fhget(struct super_block *sb, struct nfs_fh *fh, struct nfs_fattr *fattr, st
 
 	nfs_attr_check_mountpoint(sb, fattr);
 
-	if (((fattr->valid & NFS_ATTR_FATTR_FILEID) == 0) &&
-	    !nfs_attr_use_mounted_on_fileid(fattr))
+	if (nfs_attr_use_mounted_on_fileid(fattr))
+		fattr->fileid = fattr->mounted_on_fileid;
+	else if ((fattr->valid & NFS_ATTR_FATTR_FILEID) == 0)
 		goto out_no_inode;
 	if ((fattr->valid & NFS_ATTR_FATTR_TYPE) == 0)
 		goto out_no_inode;

fs/nfs/internal.h

@@ -31,8 +31,6 @@ static inline int nfs_attr_use_mounted_on_fileid(struct nfs_fattr *fattr)
 	    (((fattr->valid & NFS_ATTR_FATTR_MOUNTPOINT) == 0) &&
 	     ((fattr->valid & NFS_ATTR_FATTR_V4_REFERRAL) == 0)))
 		return 0;
-
-	fattr->fileid = fattr->mounted_on_fileid;
 	return 1;
 }