Btrfs: fix crash in scrub repair code when device is missing

Fix that when scrub tries to repair an I/O or checksum error and one of
the devices containing the mirror is missing, it crashes in bio_add_page
because the bdev is a NULL pointer for missing devices.

Reported-by: Marco L. Crociani <[e-mail]>
Signed-off-by: Stefan Behrens <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/scrub.c

···
 			page = sblock->pagev + page_index;
 			page->logical = logical;
 			page->physical = bbio->stripes[mirror_index].physical;
+			/* for missing devices, bdev is NULL */
 			page->bdev = bbio->stripes[mirror_index].dev->bdev;
 			page->mirror_num = mirror_index + 1;
 			page->page = alloc_page(GFP_NOFS);
···
 		int ret;
 		struct scrub_page *page = sblock->pagev + page_num;
 		DECLARE_COMPLETION_ONSTACK(complete);
+
+		if (page->bdev == NULL) {
+			page->io_error = 1;
+			sblock->no_io_error_seen = 0;
+			continue;
+		}
 
 		BUG_ON(!page->page);
 		bio = bio_alloc(GFP_NOFS, 1);