"Cloth is being torn"
Because nguo is a class IX (N-/N-) noun, so the verb gets the prefix i-. a- is just for animate nouns.
Why is 'the dress is being torn' = "gauni linapasuka" but 'the clothes are being torn' is "nguo inapasuliwa'? Why is one regular past tense and the other not?
Both are in the present tense, because of the na- prefix. The difference between li- and i- is because gauni is a class V (ji-/ma-) noun and nguo is a class IX (n-/n-) noun. I'm not sure why they have different endings, though.