I don't have a device large enough to test for signedness errors, since passing
huge values for start and len never make it past the i_size_read check.
However, I do see that I forgot to check the padding values, so I'll update that.

Then do you want to at least consider converting end to be of type loff_t to
match the type of value returned by i_get_size() and the type of the value
passed to truncate_inode_pages_range()? Then you only need one additional
check, something like:

/* Check for an overflow when adding start+len into end */
	if (start > (uint64_t)LLONG_MAX - len)
		return -EINVAL;

Looking at the maximum values if we have start=0 and
len=(uint64_t)LLONG_MAX we would still continue but
len=(uint64_t)LLONG_MAX+1 would return -EINVAL. Similarly for
start=(uint64_t)LLONG_MAX and len=0 it would continue
but start=(uint64_t)LLONG_MAX+1 would return -EINVAL and that 
should allow start + len to be safely added and stored into an loff_t without
overflow. With this check start+len can never be more than (uint64_t)LLONG_MAX
which would make the other checks I suggested to ensure that neither start or end
were more than (uint64_t)LLONG_MAX unnecessary.