btrfs: stop passing root argument to btrfs_add_delalloc_inodes()

There's no need to pass a root argument to btrfs_add_delalloc_inodes(), we
can just pass the inode since the root is always the root associated to
the inode in the context it's called. So remove it and have the single
caller pass only the inode.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>