btrfs: sanitizing ->fs_info, part 4

A new helper: btrfs_alloc_root(fs_info); allocates btrfs_root
and sets ->fs_info.  All places allocating the suckers converted
to it.  At that point we *never* reassign ->fs_info of btrfs_root;
it's set before anyone sees the address of newly allocated
struct btrfs_root and never assigned anywhere else.

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