Change s_fs_info to point to root inode and fs

Change s_fs_info to point to root inode and fs

We create a new data structure containing the 'fs' and the root inode
of a filesystem, and store this in the superblock.
This allows each access to that root in iget, which previously was
impossible in general.

Signed-off-by: NeilBrown <neilb@suse.de>