f2fs: accurately adjust free_sections during free_segment_range

In the Linux kernel, the following vulnerability has been resolved: f2fs: accurately adjust free_sections during free_segment_range In free_segment_range(), MAIN_SECS(sbi) is temporarily reduced by `secs` to restrict block allocation to the safe remaining main area while valid blocks in the truncated range are evacuated by GC. However, FREE_I(sbi)->free_sections tracks the total number of free sections across the whole filesystem. If any sections within the truncated range were already free upon entering free_segment_range(), failing to deduct them from free_sections causes the filesystem to overestimate available free sections in the active, reduced main area. This leads to inconsistent free section accounting during GC data migration and can trigger unexpected allocation failures or assertion errors when space is tight. Fix this by calculating the number of already-free sections in the truncated range, deducting them from free_sections upon entering free_segment_range(), and restoring them on exit.