In the case of taprio offload is not enabled, the error handling path
causes a kernel crash due to kernel NULL pointer deference.

Fix this by adding check for NULL before attempt to access 'plat->est'
on the mutex_lock() call.

The following kernel panic is observed without this patch:

RIP: 0010:mutex_lock+0x10/0x20
Call Trace:
tc_setup_taprio+0x482/0x560 [stmmac]
kmem_cache_alloc_trace+0x13f/0x490
taprio_disable_offload.isra.0+0x9d/0x180 [sch_taprio]
taprio_destroy+0x6c/0x100 [sch_taprio]
qdisc_create+0x2e5/0x4f0
tc_modify_qdisc+0x126/0x740
rtnetlink_rcv_msg+0x12b/0x380
_raw_spin_lock_irqsave+0x19/0x40
_raw_spin_unlock_irqrestore+0x18/0x30
create_object+0x212/0x340
rtnl_calcit.isra.0+0x110/0x110
netlink_rcv_skb+0x50/0x100
netlink_unicast+0x191/0x230
netlink_sendmsg+0x243/0x470
sock_sendmsg+0x5e/0x60
____sys_sendmsg+0x20b/0x280
copy_msghdr_from_user+0x5c/0x90
__mod_memcg_state+0x87/0xf0
 ___sys_sendmsg+0x7c/0xc0
lru_cache_add+0x7f/0xa0
_raw_spin_unlock+0x16/0x30
wp_page_copy+0x449/0x890
handle_mm_fault+0x921/0xfc0
__sys_sendmsg+0x59/0xa0
do_syscall_64+0x33/0x40
entry_SYSCALL_64_after_hwframe+0x44/0xa9
---[ end trace b1f19b24368a96aa ]---

Fixes: b60189e0 ("net: stmmac: Integrate EST with TAPRIO scheduler API")
Cc: <stable@vger.kernel.org> # 5.10.x
Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>