sg: disable interrupts inside sg_copy_buffer

This is the backport of the upstream commit 50bed2e2

The callers of sg_copy_buffer must disable interrupts before calling
it (since it uses kmap_atomic). Some callers use it on
interrupt-disabled code but some need to take the trouble to disable
interrupts just for this. No wonder they forget about it and we hit a
bug like:

http://bugzilla.kernel.org/show_bug.cgi?id=11529

James said that it might be better to disable interrupts inside the
function rather than risk the callers getting it wrong.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

lib/scatterlist.c

@@ -312,8 +312,9 @@ static size_t sg_copy_buffer(struct scatterlist *sgl, unsigned int nents,
 	struct scatterlist *sg;
 	size_t buf_off = 0;
 	int i;
+	unsigned long flags;
 
-	WARN_ON(!irqs_disabled());
+	local_irq_save(flags);
 
 	for_each_sg(sgl, sg, nents, i) {
 		struct page *page;
@@ -358,6 +359,8 @@ static size_t sg_copy_buffer(struct scatterlist *sgl, unsigned int nents,
 			break;
 	}
 
+	local_irq_restore(flags);
+
 	return buf_off;
 }