ufs_trunc_branch(): never call with offsets != NULL && depth2 == 0

For calls in __ufs_truncate_blocks() it's just a matter of not
incrementing offsets[0] and not making that call - immediately
following loop will be executed one extra time and we'll be just
fine.  For recursive call in ufs_trunc_branch() itself, just
assing NULL to offsets if we would be about to make such call.

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