conditional mapping of , depending on whether plugin exists

I use https://github.com/terryma/vim-smooth-scroll and so I bind <C-d> & <C-u> to ":call smooth_scroll#down(&scroll, 5, 2)<CR>" & ":call smooth_scroll#up(&scroll, 5, 2)<CR>" respectively. However I would like <C-d> & <C-u> to default to their original behaviour if the smooth scroll plugin isn't loaded.

I've tried this:

if exists('g:loaded_smoothscroll')
noremap <silent> <C-u> :call smooth_scroll#up(&scroll, 5, 2)<CR>
noremap <silent> <C-d> :call smooth_scroll#down(&scroll, 5, 2)<CR>
endif

but strangely it ignores the mapping if I load it like that. That means the exists('g:loaded_smoothscroll') check failed, but doing an :echo exists('g:loaded_smoothscroll') does indeed return 1. I'm guessing at the moment of sourcing the vimrc g:loaded_smoothscroll was 0, then it got set to 1 after after the mapping got read. How do I get around that?

If it helps I actually put the variable let g:loaded_smoothscroll = 1 into the plugin/smooth_scroll.vim file (which did not exist in the author's original version, I had to fork it myself). Is that the correct place to put the g:loaded_smoothscroll variable?

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