Btrfs: do not mark the chunk as readonly if in degraded mode

If a RAID setup has chunks that span multiple disks, and one of those
disks has failed, btrfs_chunk_readonly will return 1 since one of the
disks in that chunk's stripes is dead and therefore not writeable.  So
instead if we are in degraded mode, return 0 so we can go ahead and
allocate stuff.  Without this patch all of the block groups in a RAID1
setup will end up read-only, which will mean we can't add new disks to
the array since we won't be able to make allocations.
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/volumes.c

@@ -2538,6 +2538,11 @@ int btrfs_chunk_readonly(struct btrfs_root *root, u64 chunk_offset)
 	if (!em)
 		return 1;
 
+	if (btrfs_test_opt(root, DEGRADED)) {
+		free_extent_map(em);
+		return 0;
+	}
+
 	map = (struct map_lookup *)em->bdev;
 	for (i = 0; i < map->num_stripes; i++) {
 		if (!map->stripes[i].dev->writeable) {