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

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

Fixes: af7b3b4adda5 ("eth: bnxt: support per-queue statistics")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Link: https://patch.msgid.link/20250309134219.91670-6-ap420073@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>