syzbot reported:

    general protection fault, probably for non-canonical address 0xdffffc0000000001: 0000 [#1] PREEMPT SMP KASAN
    KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
    CPU: 0 PID: 6860 Comm: syz-executor835 Not tainted 5.9.0-rc8-syzkaller #0
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
    RIP: 0010:utf8_casefold+0x43/0x1b0 fs/unicode/utf8-core.c:107
    [...]
    Call Trace:
     f2fs_init_casefolded_name fs/f2fs/dir.c:85 [inline]
     __f2fs_setup_filename fs/f2fs/dir.c:118 [inline]
     f2fs_prepare_lookup+0x3bf/0x640 fs/f2fs/dir.c:163
     f2fs_lookup+0x10d/0x920 fs/f2fs/namei.c:494
     __lookup_hash+0x115/0x240 fs/namei.c:1445
     filename_create+0x14b/0x630 fs/namei.c:3467
     user_path_create fs/namei.c:3524 [inline]
     do_mkdirat+0x56/0x310 fs/namei.c:3664
     do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
     entry_SYSCALL_64_after_hwframe+0x44/0xa9
    [...]

The problem is that an inode has F2FS_CASEFOLD_FL set, but the
filesystem doesn't have the casefold feature flag set, and therefore
super_block::s_encoding is NULL.

Fix this by making sanity_check_inode() reject inodes that have
F2FS_CASEFOLD_FL when the filesystem doesn't have the casefold feature.

Reported-by: syzbot+05139c4039d0679e19ff@syzkaller.appspotmail.com
Fixes: 2c2eb7a3 ("f2fs: Support case-insensitive file name lookups")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>