btrfs: sanitizing ->fs_info, part 3

move assignments to ->fs_info in open_ctree() up, to the place
just after the original allocations.  Assignment for tree_root
becomes a no-op - we'd obtained fs_info from tree_root->fs_info
in the first place.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>