eth: bnxt: fix kernel panic in the bnxt_get_queue_stats{rx | tx}

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

eth: bnxt: fix kernel panic in the bnxt_get_queue_stats{rx | tx}

When qstats-get operation is executed, callbacks of netdev_stats_ops
are called. The bnxt_get_queue_stats{rx | tx} collect per-queue stats
from sw_stats in the rings.
But {rx | tx | cp}_ring are allocated when the interface is up.
So, these rings are not allocated when the interface is down.

The qstats-get is allowed even if the interface is down. However,
the bnxt_get_queue_stats{rx | tx}() accesses cp_ring and tx_ring
without null check.
So, it needs to avoid accessing rings if the interface is down.

Reproducer:
ip link set $interface down
./cli.py --spec netdev.yaml --dump qstats-get
OR
ip link set $interface down
python ./stats.py

Splat looks like:
BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 1680fa067 P4D 1680fa067 PUD 16be3b067 PMD 0
Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
CPU: 0 UID: 0 PID: 1495 Comm: python3 Not tainted 6.14.0-rc4+ #32 5cd0f999d5a15c574ac72b3e4b907341
Hardware name: ASUS System Product Name/PRIME Z690-P D4, BIOS 0603 11/01/2021
RIP: 0010:bnxt_get_queue_stats_rx+0xf/0x70 [bnxt_en]
Code: c6 87 b5 18 00 00 02 eb a2 66 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 44 01
RSP: 0018:ffffabef43cdb7e0 EFLAGS: 00010282
RAX: 0000000000000000 RBX: ffffffffc04c8710 RCX: 0000000000000000
RDX: ffffabef43cdb858 RSI: 0000000000000000 RDI: ffff8d504e850000
RBP: ffff8d506c9f9c00 R08: 0000000000000004 R09: ffff8d506bcd901c
R10: 0000000000000015 R11: ffff8d506bcd9000 R12: 0000000000000000
R13: ffffabef43cdb8c0 R14: ffff8d504e850000 R15: 0000000000000000
FS: 00007f2c5462b080(0000) GS:ffff8d575f600000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000167fd0000 CR4: 00000000007506f0
PKRU: 55555554
Call Trace:
<TASK>
? __die+0x20/0x70
? page_fault_oops+0x15a/0x460
? sched_balance_find_src_group+0x58d/0xd10
? exc_page_fault+0x6e/0x180
? asm_exc_page_fault+0x22/0x30
? bnxt_get_queue_stats_rx+0xf/0x70 [bnxt_en cdd546fd48563c280cfd30e9647efa420db07bf1]
netdev_nl_stats_by_netdev+0x2b1/0x4e0
? xas_load+0x9/0xb0
? xas_find+0x183/0x1d0
? xa_find+0x8b/0xe0
netdev_nl_qstats_get_dumpit+0xbf/0x1e0
genl_dumpit+0x31/0x90
netlink_dump+0x1a8/0x360