f2fs: get victim segment again after new cp

Previous selected segment may become free after write_checkpoint,
if we do garbage collect on this segment, and then new_curseg happen
to reuse it, it may cause f2fs_bug_on as below.

panic+0x154/0x29c
do_garbage_collect+0x15c/0xaf4
f2fs_gc+0x2dc/0x444
f2fs_balance_fs.part.22+0xcc/0x14c
f2fs_balance_fs+0x28/0x34
f2fs_map_blocks+0x5ec/0x790
f2fs_preallocate_blocks+0xe0/0x100
f2fs_file_write_iter+0x64/0x11c
new_sync_write+0xac/0x11c
vfs_write+0x144/0x1e4
SyS_write+0x60/0xc0

Here, maybe we check sit and ssa type during reset_curseg. So, we check
segment is stale or not, and select a new victim to avoid this.

Signed-off-by: Yunlei He <[e-mail]>
Signed-off-by: Jaegeuk Kim <[e-mail]>

fs/f2fs/gc.c

···
 		 * enough free sections, we should flush dent/node blocks and do
 		 * garbage collections.
 		 */
-		if (__get_victim(sbi, &segno, gc_type) || prefree_segments(sbi))
+		if (__get_victim(sbi, &segno, gc_type) ||
+						prefree_segments(sbi)) {
 			write_checkpoint(sbi, &cpc);
-		else if (has_not_enough_free_secs(sbi, 0))
+			segno = NULL_SEGNO;
+		} else if (has_not_enough_free_secs(sbi, 0)) {
 			write_checkpoint(sbi, &cpc);
+		}
 	}
 
 	if (segno == NULL_SEGNO && !__get_victim(sbi, &segno, gc_type))