syzbot has tested the proposed patch but the reproducer still triggered crash:
kernel BUG at fs/f2fs/inode.c:LINE!

F2FS-fs (loop1): invalid crc value
F2FS-fs (loop5): Magic Mismatch, valid(0xf2f52010) - read(0x0)
F2FS-fs (loop5): Can't find valid F2FS filesystem in 1th superblock
F2FS-fs (loop5): invalid crc value
------------[ cut here ]------------
kernel BUG at fs/f2fs/inode.c:238!
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 4886 Comm: syz-executor1 Not tainted 4.17.0-rc1+ #1
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:do_read_inode fs/f2fs/inode.c:238 [inline]
RIP: 0010:f2fs_iget+0x3307/0x3ca0 fs/f2fs/inode.c:313
RSP: 0018:ffff8801c44a70e8 EFLAGS: 00010293
RAX: ffff8801ce208040 RBX: ffff8801b3621080 RCX: ffffffff82eace18
F2FS-fs (loop2): Magic Mismatch, valid(0xf2f52010) - read(0x0)
RDX: 0000000000000000 RSI: ffffffff82eaf047 RDI: 0000000000000007
RBP: ffff8801c44a7410 R08: ffff8801ce208040 R09: ffffed0039ee4176
R10: ffffed0039ee4176 R11: ffff8801cf720bb7 R12: ffff8801c0efa000
R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000000
FS:  00007f753aa9d700(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
------------[ cut here ]------------
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
kernel BUG at fs/f2fs/inode.c:238!
CR2: 0000000001b03018 CR3: 00000001c8b74000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 f2fs_fill_super+0x4377/0x7bf0 fs/f2fs/super.c:2842
 mount_bdev+0x30c/0x3e0 fs/super.c:1165
 f2fs_mount+0x34/0x40 fs/f2fs/super.c:3020
 mount_fs+0xae/0x328 fs/super.c:1268
 vfs_kern_mount.part.34+0xd4/0x4d0 fs/namespace.c:1037
 vfs_kern_mount fs/namespace.c:1027 [inline]
 do_new_mount fs/namespace.c:2517 [inline]
 do_mount+0x564/0x3070 fs/namespace.c:2847
 ksys_mount+0x12d/0x140 fs/namespace.c:3063
 __do_sys_mount fs/namespace.c:3077 [inline]
 __se_sys_mount fs/namespace.c:3074 [inline]
 __x64_sys_mount+0xbe/0x150 fs/namespace.c:3074
 do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457daa
RSP: 002b:00007f753aa9cba8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000020000000 RCX: 0000000000457daa
RDX: 0000000020000000 RSI: 0000000020000100 RDI: 00007f753aa9cbf0
RBP: 0000000000000064 R08: 0000000020016a00 R09: 0000000020000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003
R13: 0000000000000064 R14: 00000000006fcb80 R15: 0000000000000000
RIP: do_read_inode fs/f2fs/inode.c:238 [inline] RSP: ffff8801c44a70e8
RIP: f2fs_iget+0x3307/0x3ca0 fs/f2fs/inode.c:313 RSP: ffff8801c44a70e8
invalid opcode: 0000 [#2] SMP KASAN
---[ end trace 1cbcbec2156680bc ]---

Reported-and-tested-by: syzbot+41a1b341571f0952badb@syzkaller.appspotmail.com
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>