lib: bitmap: fix the duplicated comments on bitmap_to_arr64()

Thanks to the recent commit 0a97953f

 ("lib: add
bitmap_{from,to}_arr64") now we can directly convert a U64 value into a
bitmap and vice verse.

However when checking the header there is duplicated helper for
bitmap_to_arr64(), but no bitmap_from_arr64().

Just fix the copy-n-paste error.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>