Due to the conditional here:

I think it could be rewritten as something like:

tmp[0] ^= 135 & byte(-(in[15] >> 7))

(untested)

Now the broader question is, does this actually matter? In a disk-encryption scenario, an attacker who can time chosen writes (or observe ciphertexts along with timing info) could perhaps infer a few bits of L and use that as a device fingerprinting vector, even if there's no way to break the security of the scheme as a whole.

Edit: For some reason I completely glossed over the second conditional that does the carry propagation - same issue there.