xen-blkback: check the number of iovecs before allocating a bios

With the introduction of indirect segments we can receive requests
with a number of segments bigger than the maximum number of allowed
iovecs in a bios, so make sure that blkback doesn't try to allocate a
bios with more iovecs than BIO_MAX_PAGES

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/blkback.c

@@ -1247,7 +1247,8 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 				     seg[i].nsec << 9,
 				     seg[i].offset) == 0)) {
 
-			bio = bio_alloc(GFP_KERNEL, nseg-i);
+			int nr_iovecs = min_t(int, (nseg-i), BIO_MAX_PAGES);
+			bio = bio_alloc(GFP_KERNEL, nr_iovecs);
 			if (unlikely(bio == NULL))
 				goto fail_put_bio;