sfc: adjust efx->xdp_tx_queue_count with the real number of initialized queues

efx->xdp_tx_queue_count is initially initialized to num_possible_cpus() and is
later used to allocate and traverse efx->xdp_tx_queues lookup array. However,
we may end up not initializing all the array slots with real queues during
probing. This results, for example, in a NULL pointer dereference, when running
"# ethtool -S <iface>", similar to below

[2570283.664955][T4126959] BUG: kernel NULL pointer dereference, address: 00000000000000f8
[2570283.681283][T4126959] #PF: supervisor read access in kernel mode
[2570283.695678][T4126959] #PF: error_code(0x0000) - not-present page
[2570283.710013][T4126959] PGD 0 P4D 0
[2570283.721649][T4126959] Oops: 0000 [#1] SMP PTI
[2570283.734108][T4126959] CPU: 23 PID: 4126959 Comm: ethtool Tainted: G           O      5.10.20-cloudflare-2021.3.1 #1
[2570283.752641][T4126959] Hardware name: <redacted>
[2570283.781408][T4126959] RIP: 0010:efx_ethtool_get_stats+0x2ca/0x330 [sfc]
[2570283.796073][T4126959] Code: 00 85 c0 74 39 48 8b 95 a8 0f 00 00 48 85 d2 74 2d 31 c0 eb 07 48 8b 95 a8 0f 00 00 48 63 c8 49 83 c4 08 83 c0 01 48 8b 14 ca <48> 8b 92 f8 00 00 00 49 89 54 24 f8 39 85 a0 0f 00 00 77 d7 48 8b
[2570283.831259][T4126959] RSP: 0018:ffffb79a77657ce8 EFLAGS: 00010202
[2570283.845121][T4126959] RAX: 0000000000000019 RBX: ffffb799cd0c9280 RCX: 0000000000000018
[2570283.860872][T4126959] RDX: 0000000000000000 RSI: ffff96dd970ce000 RDI: 0000000000000005
[2570283.876525][T4126959] RBP: ffff96dd86f0a000 R08: ffff96dd970ce480 R09: 000000000000005f
[2570283.892014][T4126959] R10: ffffb799cd0c9fff R11: ffffb799cd0c9000 R12: ffffb799cd0c94f8
[2570283.907406][T4126959] R13: ffffffffc11b1090 R14: ffff96dd970ce000 R15: ffffffffc11cd66c
[2570283.922705][T4126959] FS:  00007fa7723f8740(0000) GS:ffff96f51fac0000(0000) knlGS:0000000000000000
[2570283.938848][T4126959] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[2570283.952524][T4126959] CR2: 00000000000000f8 CR3: 0000001a73e6e006 CR4: 00000000007706e0
[2570283.967529][T4126959] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[2570283.982400][T4126959] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[2570283.997308][T4126959] PKRU: 55555554
[25702840.007649][T4126959] Call Trace:
[25702840.017598][T4126959]  dev_ethtool+0x1832/0x2830

Fix this by adjusting efx->xdp_tx_queue_count after probing to reflect the true
value of initialized slots in efx->xdp_tx_queues.

Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
Fixes: e26ca4b5 ("sfc: reduce the number of requested xdp ev queues")
Cc: <stable@vger.kernel.org> # 5.12.x
Signed-off-by: David S. Miller <davem@davemloft.net>