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

In the Linux kernel, the following vulnerability has been resolved:

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.