ufs_trunc_..indirect(): more massage towards unifying

Instead of manually checking that the array contains only zeroes,
find the position of the last non-zero (in __ufs_truncate(), where
we can conveniently do that) and use that to tell if there's
any non-zero in the array tail passed to ufs_trunc_...indirect().

The goal of all that clumsiness is to get fold these functions
together.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>