Btrfs: deal with bad mappings in btrfs_map_block

Martin Steigerwald reported a BUG_ON() in btrfs_map_block where we didn't find
a chunk for a particular block we were trying to map.  This happened because the
block was bogus.  We shouldn't be BUG_ON()'ing in this case, just print a
message and return an error.  This came from reada_add_block and it appears to
deal with an error fine so we should be good there.  Thanks,
Reported-by: Martin Steigerwald <Martin@lichtvoll.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/volumes.c

@@ -4406,10 +4406,16 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info, int rw,
 		btrfs_crit(fs_info, "unable to find logical %llu len %llu",
 			(unsigned long long)logical,
 			(unsigned long long)*length);
-		BUG();
+		return -EINVAL;
+	}
+
+	if (em->start > logical || em->start + em->len < logical) {
+		btrfs_crit(fs_info, "found a bad mapping, wanted %Lu, "
+			   "found %Lu-%Lu\n", logical, em->start,
+			   em->start + em->len);
+		return -EINVAL;
 	}
 
-	BUG_ON(em->start > logical || em->start + em->len < logical);
 	map = (struct map_lookup *)em->bdev;
 	offset = logical - em->start;