On older chips not supporting multiple RSS contexts, reducing
ethtool channels will crash:

BUG: kernel NULL pointer dereference, address: 00000000000000b8
PGD 0 P4D 0
Oops: Oops: 0000 [#1] PREEMPT SMP PTI
CPU: 1 PID: 7032 Comm: ethtool Tainted: G S                 6.10.0-rc4 #1
Hardware name: Dell Inc. PowerEdge R730/072T6D, BIOS 2.4.3 01/17/2017
RIP: 0010:bnxt_get_max_rss_ctx_ring+0x4c/0x90 [bnxt_en]
Code: c3 d3 eb 4c 8b 83 38 01 00 00 48 8d bb 38 01 00 00 4c 39 c7 74 42 41 8d 54 24 ff 31 c0 0f b7 d2 4c 8d 4c 12 02 66 85 ed 74 1d <49> 8b 90 b8 00 00 00 49 8d 34 11 0f b7 0a 66 39 c8 0f 42 c1 48 83
RSP: 0018:ffffaaa501d23ba8 EFLAGS: 00010202
RAX: 0000000000000000 RBX: ffff8efdf600c940 RCX: 0000000000000000
RDX: 000000000000007f RSI: ffffffffacf429c4 RDI: ffff8efdf600ca78
RBP: 0000000000000080 R08: 0000000000000000 R09: 0000000000000100
R10: 0000000000000001 R11: ffffaaa501d238c0 R12: 0000000000000080
R13: 0000000000000000 R14: ffff8efdf600c000 R15: 0000000000000006
FS:  00007f977a7d2740(0000) GS:ffff8f041f840000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000000b8 CR3: 00000002320aa004 CR4: 00000000003706f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
? __die_body+0x15/0x60
? page_fault_oops+0x157/0x440
? do_user_addr_fault+0x60/0x770
? _raw_spin_lock_irqsave+0x12/0x40
? exc_page_fault+0x61/0x120
? asm_exc_page_fault+0x22/0x30
? bnxt_get_max_rss_ctx_ring+0x4c/0x90 [bnxt_en]
? bnxt_get_max_rss_ctx_ring+0x25/0x90 [bnxt_en]
bnxt_set_channels+0x9d/0x340 [bnxt_en]
ethtool_set_channels+0x14b/0x210
__dev_ethtool+0xdf8/0x2890
? preempt_count_add+0x6a/0xa0
? percpu_counter_add_batch+0x23/0x90
? filemap_map_pages+0x417/0x4a0
? avc_has_extended_perms+0x185/0x420
? __pfx_udp_ioctl+0x10/0x10
? sk_ioctl+0x55/0xf0
? kmalloc_trace_noprof+0xe0/0x210
? dev_ethtool+0x54/0x170
dev_ethtool+0xa2/0x170
dev_ioctl+0xbe/0x530
sock_do_ioctl+0xa3/0xf0
sock_ioctl+0x20d/0x2e0

bp->rss_ctx_list is not initialized if the chip or firmware does not
support multiple RSS contexts.  Fix it by adding a check in
bnxt_get_max_rss_ctx_ring() before proceeding to reference
bp->rss_ctx_list.

Fixes: 0d1b7d6c9274 ("bnxt: fix crashes when reducing ring count with active RSS contexts")
Reported-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/netdev/ZpFEJeNpwxW1aW9k@gmail.com/
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20240712175318.166811-1-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>