btrfs: embed delayed root to struct btrfs_fs_info

btrfs: embed delayed root to struct btrfs_fs_info

The fs_info::delayed_root is allocated dynamically but there's only one
instance per filesystem so we can embed it into the fs_info itself.
The two object have the same lifetime and delayed roots are always
present so we don't need to allocate it on demand from slab.

There's still some space left in fs_info until the 4K so there won't be
an spill over to next page on release config (size grows from 3880 to
3952). In case we want to shrink fs_info there are still holes to fill
or we can separate other non-core or optional structures if needed.

Link: https://lore.kernel.org/all/cover.1767979013.git.dsterba@suse.com/
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>