f2fs: optimize representative type determination in GC

In large section mode, do_garbage_collect() previously determined the
section's representative type by looking only at the first segment of
the section. However, if data was fsynced into an area previously used
as a node section, and this area is recovered during roll-forward
recovery after sudden power off (SPO), GC would incorrectly assume the
section's type based on an empty or obsolete first segment. This caused
the recovered data segment to be misunderstood as being stuck inside a
node section, triggering false inconsistency panics (Inconsistent
segment type in SSA and SIT) and subsequent mount failures.

This patch optimizes do_garbage_collect() to determine the section's
representative type by identifying the first segment that actually
contains valid blocks (valid_blocks > 0) during the main GC loop. This
eliminates false alarms from empty/obsolete leading segments while
maintaining strict section-level type consistency checks for genuine
corruption.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

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