btrfs: Prevent possible ERR_PTR() dereference

In btrfs_full_stripe_len/btrfs_is_parity_mirror we have similar code which
gets the chunk map for a particular range via get_chunk_map. However,
get_chunk_map can return an ERR_PTR value and while the 2 callers do catch
this with a WARN_ON they then proceed to indiscriminately dereference the
extent map. This of course leads to a crash. Fix the offenders by making the
dereference conditional on IS_ERR.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -5173,12 +5173,13 @@ unsigned long btrfs_full_stripe_len(struct btrfs_fs_info *fs_info,
 	unsigned long len = fs_info->sectorsize;
 
 	em = get_chunk_map(fs_info, logical, len);
-	WARN_ON(IS_ERR(em));
 
+	if (!WARN_ON(IS_ERR(em))) {
 		map = em->map_lookup;
 		if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK)
 			len = map->stripe_len * nr_data_stripes(map);
 		free_extent_map(em);
+	}
 	return len;
 }
 
@@ -5190,12 +5191,13 @@ int btrfs_is_parity_mirror(struct btrfs_fs_info *fs_info,
 	int ret = 0;
 
 	em = get_chunk_map(fs_info, logical, len);
-	WARN_ON(IS_ERR(em));
 
+	if(!WARN_ON(IS_ERR(em))) {
 		map = em->map_lookup;
 		if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK)
 			ret = 1;
 		free_extent_map(em);
+	}
 	return ret;
 }