f2fs: reduce searching region of segmap when set free section

In __set_free we will check whether all segment are free in one section
when free one segment, in order to set section to free status. But the
searching region of segmap is from start segno to last segno of main
area, it's not necessary. So let's just only check all segment bitmap
of target section.
Signed-off-by: NWanpeng Li <wanpeng.li@linux.intel.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.h

@@ -336,7 +336,8 @@ static inline void __set_free(struct f2fs_sb_info *sbi, unsigned int segno)
 	clear_bit(segno, free_i->free_segmap);
 	free_i->free_segments++;
 
-	next = find_next_bit(free_i->free_segmap, MAIN_SEGS(sbi), start_segno);
+	next = find_next_bit(free_i->free_segmap,
+			start_segno + sbi->segs_per_sec, start_segno);
 	if (next >= start_segno + sbi->segs_per_sec) {
 		clear_bit(secno, free_i->free_secmap);
 		free_i->free_sections++;