fsck.f2fs: Split build_segment_manager() to speed up auto-fix am: b5cd946910 am: 7fbab69c44

Original change: https://googleplex-android-review.googlesource.com/c/platform/external/f2fs-tools/+/12056203

Change-Id: Iaa0c89a454e706c88e6737d9ceb345e1837fbe37