btrfs-progs: convert: Prevent accounting blocks beyond end of device

btrfs-progs: convert: Prevent accounting blocks beyond end of device

When looping across data block bitmap, __ext2_add_one_block() may add
blocks which do not exist on the underlying disk. This commit prevents
this from happening by checking the block index against the maximum
block count that was present in the ext4 filesystem instance that is
being converted.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>