f2fs: fix to avoid return invalid mtime from f2fs_get_section_mtime()

syzbot reported a f2fs bug as below:

------------[ cut here ]------------
kernel BUG at fs/f2fs/gc.c:373!
CPU: 0 UID: 0 PID: 5316 Comm: syz.0.0 Not tainted 6.13.0-rc3-syzkaller-00044-gaef25be35d23 #0
RIP: 0010:get_cb_cost fs/f2fs/gc.c:373 [inline]
RIP: 0010:get_gc_cost fs/f2fs/gc.c:406 [inline]
RIP: 0010:f2fs_get_victim+0x68b1/0x6aa0 fs/f2fs/gc.c:912
Call Trace:
 <TASK>
 __get_victim fs/f2fs/gc.c:1707 [inline]
 f2fs_gc+0xc89/0x2f60 fs/f2fs/gc.c:1915
 f2fs_ioc_gc fs/f2fs/file.c:2624 [inline]
 __f2fs_ioctl+0x4cc9/0xb8b0 fs/f2fs/file.c:4482
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

w/ below testcase, it can reproduce directly:
- dd if=/dev/zero of=/tmp/file bs=1M count=64
- mkfs.f2fs /tmp/file
- mount -t f2fs -o loop,mode=fragment:block /tmp/file /mnt/f2fs
- echo 0 >  /sys/fs/f2fs/loop0/min_ssr_sections
- dd if=/dev/zero of=/mnt/f2fs/file bs=1M count=5
- umount /mnt/f2fs
- for((i=4096;i<16384;i+=512)) do inject.f2fs --sit 0 --blk $i --mb mtime --val -1 /tmp/file; done
- mount -o loop /tmp/file /mnt/f2fs
- f2fs_io gc 0 /mnt/f2fs/file

static unsigned int get_cb_cost()
{
	...
	mtime = f2fs_get_section_mtime(sbi, segno);
	f2fs_bug_on(sbi, mtime == INVALID_MTIME);
	...
}

The root cause is: mtime in f2fs_sit_entry can be fuzzed to INVALID_MTIME,
then it will trigger BUG_ON in get_cb_cost() during GC.

Let's change behavior of f2fs_get_section_mtime() as below for fix:
- return INVALID_MTIME only if total valid blocks is zero.
- return INVALID_MTIME - 1 if average mtime calculated is
INVALID_MTIME.

Fixes: b19ee72 ("f2fs: introduce f2fs_get_section_mtime")
Reported-by: syzbot+b9972806adbe20a910eb@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-f2fs-devel/6768c82e.050a0220.226966.0035.GAE@google.com
Cc: liuderong <liuderong@oppo.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: chaseyu

fs/f2fs/segment.c

@@ -5549,8 +5549,10 @@ unsigned long long f2fs_get_section_mtime(struct f2fs_sb_info *sbi,
 	secno = GET_SEC_FROM_SEG(sbi, segno);
 	start = GET_SEG_FROM_SEC(sbi, secno);
 
-	if (!__is_large_section(sbi))
-		return get_seg_entry(sbi, start + i)->mtime;
+	if (!__is_large_section(sbi)) {
+		mtime = get_seg_entry(sbi, start + i)->mtime;
+		goto out;
+	}
 
 	for (i = 0; i < usable_segs_per_sec; i++) {
 		/* for large section, only check the mtime of valid segments */
@@ -5563,7 +5565,11 @@ unsigned long long f2fs_get_section_mtime(struct f2fs_sb_info *sbi,
 	if (total_valid_blocks == 0)
 		return INVALID_MTIME;
 
-	return div_u64(mtime, total_valid_blocks);
+	mtime = div_u64(mtime, total_valid_blocks);
+out:
+	if (unlikely(mtime == INVALID_MTIME))
+		mtime -= 1;
+	return mtime;
 }
 
 /*