f2fs: skip clean segment for gc

If a segment in a section is clean or prefreed, we don't need to get its summary
and do gc.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -795,6 +795,10 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
 	}
 
 	for (segno = start_segno; segno < end_segno; segno++) {
+
+		if (get_valid_blocks(sbi, segno, 1) == 0)
+			continue;
+
 		/* find segment summary of victim */
 		sum_page = find_get_page(META_MAPPING(sbi),
 					GET_SUM_BLOCK(sbi, segno));