f2fs: fix to avoid NULL pointer dereference in f2fs_check_quota_consistency()

syzbot reported a f2fs bug as below:

Oops: gen[  107.736417][ T5848] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 1 UID: 0 PID: 5848 Comm: syz-executor263 Tainted: G        W           6.17.0-rc1-syzkaller-00014-g0e39a731820a #0 PREEMPT_{RT,(full)}
RIP: 0010:strcmp+0x3c/0xc0 lib/string.c:284
Call Trace:
 <TASK>
 f2fs_check_quota_consistency fs/f2fs/super.c:1188 [inline]
 f2fs_check_opt_consistency+0x1378/0x2c10 fs/f2fs/super.c:1436
 __f2fs_remount fs/f2fs/super.c:2653 [inline]
 f2fs_reconfigure+0x482/0x1770 fs/f2fs/super.c:5297
 reconfigure_super+0x224/0x890 fs/super.c:1077
 do_remount fs/namespace.c:3314 [inline]
 path_mount+0xd18/0xfe0 fs/namespace.c:4112
 do_mount fs/namespace.c:4133 [inline]
 __do_sys_mount fs/namespace.c:4344 [inline]
 __se_sys_mount+0x317/0x410 fs/namespace.c:4321
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The direct reason is f2fs_check_quota_consistency() may suffer null-ptr-deref
issue in strcmp().

The bug can be reproduced w/ below scripts:
mkfs.f2fs -f /dev/vdb
mount -t f2fs -o usrquota /dev/vdb /mnt/f2fs
quotacheck -uc /mnt/f2fs/
umount /mnt/f2fs
mount -t f2fs -o usrjquota=aquota.user,jqfmt=vfsold /dev/vdb /mnt/f2fs
mount -t f2fs -o remount,usrjquota=,jqfmt=vfsold /dev/vdb /mnt/f2fs
umount /mnt/f2fs

So, before old_qname and new_qname comparison, we need to check whether
they are all valid pointers, fix it.

Reported-by: syzbot+d371efea57d5aeab877b@syzkaller.appspotmail.com
Fixes: d185351 ("f2fs: separate the options parsing and options checking")
Closes: https://lore.kernel.org/linux-f2fs-devel/689ff889.050a0220.e29e5.0037.GAE@google.com
Cc: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: chaseyu

fs/f2fs/super.c

@@ -1219,7 +1219,8 @@ static int f2fs_check_quota_consistency(struct fs_context *fc,
 				goto err_jquota_change;
 
 			if (old_qname) {
-				if (strcmp(old_qname, new_qname) == 0) {
+				if (new_qname &&
+					strcmp(old_qname, new_qname) == 0) {
 					ctx->qname_mask &= ~(1 << i);
 					continue;
 				}