Btrfs: Fix NFS exporting of subvol roots.

Date: Tue, 19 Aug 2008 22:20:17 +0100
btrfs_lookup_fs_root() only finds subvol roots which have already been
seen and put into the cache. For btrfs_get_dentry() we actually have to
go to the medium -- so use btrfs_read_fs_root_no_name() instead.

In btrfs_get_parent(), notice when we've hit the root of the
subvolume and return the real root instead.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>