btrfs-progs: convert: add compatibility layer for e2fsprogs < 1.42

btrfs-progs: convert: add compatibility layer for e2fsprogs < 1.42

The new convert framework copies code from current dumpe2fs, which uses
BIGALLOC feature introduced in e2fsprogs v1.42.

While there are a lot of enterprise distributions which are still using
v1.41 e2fsprogs, this will cause compile error for them.

This patch introduces backward compatibility for new convert framework,
by manually introduce macros for ext2 BIGALLOC feature.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>