fix writev regression: pan hanging unkillable and un-straceable

patch 124d3b70 in mainline.

Frederik Himpe reported an unkillable and un-straceable pan process.

Zero length iovecs can go into an infinite loop in writev, because the 
iovec iterator does not always advance over them.

The sequence required to trigger this is not trivial. I think it 
requires that a zero-length iovec be followed by a non-zero-length iovec 
which causes a pagefault in the atomic usercopy. This causes the writev 
code to drop back into single-segment copy mode, which then tries to 
copy the 0 bytes of the zero-length iovec; a zero length copy looks like 
a failure though, so it loops.

Put a test into iov_iter_advance to catch zero-length iovecs. We could 
just put the test in the fallback path, but I feel it is more robust to 
skip over zero-length iovecs throughout the code (iovec iterator may be 
used in filesystems too, so it should be robust).

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>