- writev currently returns -EFAULT if _any_ of the segments has an
invalid address.  We should only return -EFAULT if the first segment
has a bad address.

If some of the first segments have valid addresses we need to write
them and return a partial result.

- The current code only checks if the sum-of-lengths is negative.  If
individual segments have a negative length but the result is positive
we miss that.

So rework the code to detect this, and to be immune to odd wrapping
situations.

As a bonus, we save one pass across the iovec.

- ditto for readv.

The check for "does any segment have a negative length" has already
been performed in do_readv_writev(), but it's basically free here, and
we need to do it for generic_file_read/write anyway.

This all means that the iov_length() function is unsafe because of
wrap/overflow isues.  It should only be used after the
generic_file_read/write or do_readv_writev() checking has been
performed.  Its callers have been reviewed and they are OK.

The code now passes LTP testing and has been QA'd by Janet's team.