Btrfs: convert BUG_ON to WARN_ON

These two BUG_ON()s would never be true, ensured by callers' logic.
Reviewed-by: NQu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: NLiu Bo <bo.li.liu@oracle.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -5174,7 +5174,7 @@ 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);
-	BUG_ON(IS_ERR(em));
+	WARN_ON(IS_ERR(em));
 
 	map = em->map_lookup;
 	if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK)
@@ -5191,7 +5191,7 @@ int btrfs_is_parity_mirror(struct btrfs_fs_info *fs_info,
 	int ret = 0;
 
 	em = get_chunk_map(fs_info, logical, len);
-	BUG_ON(IS_ERR(em));
+	WARN_ON(IS_ERR(em));
 
 	map = em->map_lookup;
 	if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK)