Btrfs-progs: fsck: deal with snapshot one by one when rebuilding extent tree

Btrfs-progs: fsck: deal with snapshot one by one when rebuilding extent tree

Previously, we deal with node block firstly and then leaf block which can
maximize readahead. However, to rebuild extent tree, we need deal with snapshot
one by one.

This patch makes us deal with snapshot one by one if we need rebuild extent
tree otherwise we drop into previous way.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Wang Shilong <wangshilong1991@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>