fix a kmap leak in virtio_console

While we are at it, don't do kmap() under kmap_atomic(), *especially*
for a page we'd allocated with GFP_KERNEL.  It's spelled "page_address",
and had that been more than that, we'd have a real trouble - kmap_high()
can block, and doing that while holding kmap_atomic() is a Bad Idea(tm).
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/char/virtio_console.c

@@ -890,12 +890,10 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 	} else {
 		/* Failback to copying a page */
 		struct page *page = alloc_page(GFP_KERNEL);
-		char *src = buf->ops->map(pipe, buf, 1);
-		char *dst;
+		char *src;
 
 		if (!page)
 			return -ENOMEM;
-		dst = kmap(page);
 
 		offset = sd->pos & ~PAGE_MASK;
 
@@ -903,9 +901,8 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 		if (len + offset > PAGE_SIZE)
 			len = PAGE_SIZE - offset;
 
-		memcpy(dst + offset, src + buf->offset, len);
-
-		kunmap(page);
+		src = buf->ops->map(pipe, buf, 1);
+		memcpy(page_address(page) + offset, src + buf->offset, len);
 		buf->ops->unmap(pipe, buf, src);
 
 		sg_set_page(&(sgl->sg[sgl->n]), page, len, offset);