Fix thinko in iov_iter_single_seg_count

The branches of the if (i->type & ITER_BVEC) statement in
iov_iter_single_seg_count() are the wrong way around; if ITER_BVEC is
clear then we use i->bvec, when we should be using i->iov.  This fixes
it.

In my case, the symptom that this caused was that a KVM guest doing
filesystem operations on a virtual disk would result in one of qemu's
threads on the host going into an infinite loop in
generic_perform_write().  The loop would hit the copied == 0 case and
call iov_iter_single_seg_count() to reduce the number of bytes to try
to process, but because of the error, iov_iter_single_seg_count()
would just return i->count and the loop made no progress and continued
forever.

Cc: stable@vger.kernel.org # 3.16+
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

mm/iov_iter.c

@@ -911,9 +911,9 @@ size_t iov_iter_single_seg_count(const struct iov_iter *i)
 	if (i->nr_segs == 1)
 		return i->count;
 	else if (i->type & ITER_BVEC)
-		return min(i->count, i->iov->iov_len - i->iov_offset);
-	else
 		return min(i->count, i->bvec->bv_len - i->iov_offset);
+	else
+		return min(i->count, i->iov->iov_len - i->iov_offset);
 }
 EXPORT_SYMBOL(iov_iter_single_seg_count);