sch_dsmark: fix a NULL deref in qdisc_reset()

If Qdisc_ops->init() is failed, Qdisc_ops->reset() would be called.
When dsmark_init(Qdisc_ops->init()) is failed, it possibly doesn't
initialize dsmark_qdisc_data->q. But dsmark_reset(Qdisc_ops->reset())
uses dsmark_qdisc_data->q pointer wihtout any null checking.
So, panic would occur.

Test commands:
    sysctl net.core.default_qdisc=dsmark -w
    ip link add dummy0 type dummy
    ip link add vw0 link dummy0 type virt_wifi
    ip link set vw0 up

Splat looks like:
KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
CPU: 3 PID: 684 Comm: ip Not tainted 5.12.0+ #910
RIP: 0010:qdisc_reset+0x2b/0x680
Code: 1f 44 00 00 48 b8 00 00 00 00 00 fc ff df 41 57 41 56 41 55 41 54
55 48 89 fd 48 83 c7 18 53 48 89 fa 48 c1 ea 03 48 83 ec 20 <80> 3c 02
00 0f 85 09 06 00 00 4c 8b 65 18 0f 1f 44 00 00 65 8b 1d
RSP: 0018:ffff88800fda6bf8 EFLAGS: 00010282
RAX: dffffc0000000000 RBX: ffff8880050ed800 RCX: 0000000000000000
RDX: 0000000000000003 RSI: ffffffff99e34100 RDI: 0000000000000018
RBP: 0000000000000000 R08: fffffbfff346b553 R09: fffffbfff346b553
R10: 0000000000000001 R11: fffffbfff346b552 R12: ffffffffc0824940
R13: ffff888109e83800 R14: 00000000ffffffff R15: ffffffffc08249e0
FS:  00007f5042287680(0000) GS:ffff888119800000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055ae1f4dbd90 CR3: 0000000006760002 CR4: 00000000003706e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 ? rcu_read_lock_bh_held+0xa0/0xa0
 dsmark_reset+0x3d/0xf0 [sch_dsmark]
 qdisc_reset+0xa9/0x680
 qdisc_destroy+0x84/0x370
 qdisc_create_dflt+0x1fe/0x380
 attach_one_default_qdisc.constprop.41+0xa4/0x180
 dev_activate+0x4d5/0x8c0
 ? __dev_open+0x268/0x390
 __dev_open+0x270/0x390

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>