f2fs: fix to check segment boundary during SIT page readahead

As syzbot reported:

kernel BUG at fs/f2fs/segment.h:657!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 16220 Comm: syz-executor.0 Not tainted 5.9.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:f2fs_ra_meta_pages+0xa51/0xdc0 fs/f2fs/segment.h:657
Call Trace:
 build_sit_entries fs/f2fs/segment.c:4195 [inline]
 f2fs_build_segment_manager+0x4b8a/0xa3c0 fs/f2fs/segment.c:4779
 f2fs_fill_super+0x377d/0x6b80 fs/f2fs/super.c:3633
 mount_bdev+0x32e/0x3f0 fs/super.c:1417
 legacy_get_tree+0x105/0x220 fs/fs_context.c:592
 vfs_get_tree+0x89/0x2f0 fs/super.c:1547
 do_new_mount fs/namespace.c:2875 [inline]
 path_mount+0x1387/0x2070 fs/namespace.c:3192
 do_mount fs/namespace.c:3205 [inline]
 __do_sys_mount fs/namespace.c:3413 [inline]
 __se_sys_mount fs/namespace.c:3390 [inline]
 __x64_sys_mount+0x27f/0x300 fs/namespace.c:3390
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

@blkno in f2fs_ra_meta_pages could exceed max segment count, causing panic
in following sanity check in current_sit_addr(), add check condition to
avoid this issue.

Reported-by: syzbot+3698081bcf0bb2d12174@syzkaller.appspotmail.com
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/checkpoint.c

@@ -243,6 +243,8 @@ int f2fs_ra_meta_pages(struct f2fs_sb_info *sbi, block_t start, int nrpages,
 					blkno * NAT_ENTRY_PER_BLOCK);
 			break;
 		case META_SIT:
+			if (unlikely(blkno >= TOTAL_SEGS(sbi)))
+				goto out;
 			/* get sit block addr */
 			fio.new_blkaddr = current_sit_addr(sbi,
 					blkno * SIT_ENTRY_PER_BLOCK);