virtio_vdpa: build affinity masks conditionally

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

virtio_vdpa: build affinity masks conditionally

We try to build affinity mask via create_affinity_masks()
unconditionally which may lead several issues:

- the affinity mask is not used for parent without affinity support
(only VDUSE support the affinity now)
- the logic of create_affinity_masks() might not work for devices
other than block. For example it's not rare in the networking device
where the number of queues could exceed the number of CPUs. Such
case breaks the current affinity logic which is based on
group_cpus_evenly() who assumes the number of CPUs are not less than
the number of groups. This can trigger a warning[1]:

if (ret >= 0)
WARN_ON(nr_present + nr_others < numgrps);

Fixing this by only build the affinity masks only when

- Driver passes affinity descriptor, driver like virtio-blk can make
sure to limit the number of queues when it exceeds the number of CPUs
- Parent support affinity setting config ops

This help to avoid the warning. More optimizations could be done on
top.

[1]
[ 682.146655] WARNING: CPU: 6 PID: 1550 at lib/group_cpus.c:400 group_cpus_evenly+0x1aa/0x1c0
[ 682.146668] CPU: 6 PID: 1550 Comm: vdpa Not tainted 6.5.0-rc5jason+ #79
[ 682.146671] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
[ 682.146673] RIP: 0010:group_cpus_evenly+0x1aa/0x1c0
[ 682.146676] Code: 4c 89 e0 5b 5d 41 5c 41 5d 41 5e c3 cc cc cc cc e8 1b c4 74 ff 48 89 ef e8 13 ac 98 ff 4c 89 e7 45 31 e4 e8 08 ac 98 ff eb c2 <0f> 0b eb b6 e8 fd 05 c3 00 45 31 e4 eb e5 cc cc cc cc cc cc cc cc
[ 682.146679] RSP: 0018:ffffc9000215f498 EFLAGS: 00010293
[ 682.146682] RAX: 000000000001f1e0 RBX: 0000000000000041 RCX: 0000000000000000
[ 682.146684] RDX: ffff888109922058 RSI: 0000000000000041 RDI: 0000000000000030
[ 682.146686] RBP: ffff888109922058 R08: ffffc9000215f498 R09: ffffc9000215f4a0
[ 682.146687] R10: 00000000000198d0 R11: 0000000000000030 R12: ffff888107e02800
[ 682.146689] R13: 0000000000000030 R14: 0000000000000030 R15: 0000000000000041
[ 682.146692] FS: 00007fef52315740(0000) GS:ffff888237380000(0000) knlGS:0000000000000000
[ 682.146695] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 682.146696] CR2: 00007fef52509000 CR3: 0000000110dbc004 CR4: 0000000000370ee0
[ 682.146698] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 682.146700] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 682.146701] Call Trace:
[ 682.146703] <TASK>
[ 682.146705] ? __warn+0x7b/0x130
[ 682.146709] ? group_cpus_evenly+0x1aa/0x1c0
[ 682.146712] ? report_bug+0x1c8/0x1e0
[ 682.146717] ? handle_bug+0x3c/0x70
[ 682.146721] ? exc_invalid_op+0x14/0x70
[ 682.146723] ? asm_exc_invalid_op+0x16/0x20
[ 682.146727] ? group_cpus_evenly+0x1aa/0x1c0
[ 682.146729] ? group_cpus_evenly+0x15c/0x1c0
[ 682.146731] create_affinity_masks+0xaf/0x1a0
[ 682.146735] virtio_vdpa_find_vqs+0x83/0x1d0
[ 682.146738] ? __pfx_default_calc_sets+0x10/0x10
[ 682.146742] virtnet_find_vqs+0x1f0/0x370
[ 682.146747] virtnet_probe+0x501/0xcd0
[ 682.146749] ? vp_modern_get_status+0x12/0x20
[ 682.146751] ? get_cap_addr.isra.0+0x10/0xc0
[ 682.146754] virtio_dev_probe+0x1af/0x260
[ 682.146759] really_probe+0x1a5/0x410