netfilter: nft_set_hash: unaligned atomic read on struct nft_set_ext

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

netfilter: nft_set_hash: unaligned atomic read on struct nft_set_ext

Access to genmask field in struct nft_set_ext results in unaligned
atomic read:

[ 72.130109] Unable to handle kernel paging request at virtual address ffff0000c2bb708c
[ 72.131036] Mem abort info:
[ 72.131213] ESR = 0x0000000096000021
[ 72.131446] EC = 0x25: DABT (current EL), IL = 32 bits
[ 72.132209] SET = 0, FnV = 0
[ 72.133216] EA = 0, S1PTW = 0
[ 72.134080] FSC = 0x21: alignment fault
[ 72.135593] Data abort info:
[ 72.137194] ISV = 0, ISS = 0x00000021, ISS2 = 0x00000000
[ 72.142351] CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[ 72.145989] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[ 72.150115] swapper pgtable: 4k pages, 48-bit VAs, pgdp=0000000237d27000
[ 72.154893] [ffff0000c2bb708c] pgd=0000000000000000, p4d=180000023ffff403, pud=180000023f84b403, pmd=180000023f835403,
+pte=0068000102bb7707
[ 72.163021] Internal error: Oops: 0000000096000021 [#1] SMP
[...]
[ 72.170041] CPU: 7 UID: 0 PID: 54 Comm: kworker/7:0 Tainted: G E 6.13.0-rc3+ #2
[ 72.170509] Tainted: [E]=UNSIGNED_MODULE
[ 72.170720] Hardware name: QEMU QEMU Virtual Machine, BIOS edk2-stable202302-for-qemu 03/01/2023
[ 72.171192] Workqueue: events_power_efficient nft_rhash_gc [nf_tables]
[ 72.171552] pstate: 21400005 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
[ 72.171915] pc : nft_rhash_gc+0x200/0x2d8 [nf_tables]
[ 72.172166] lr : nft_rhash_gc+0x128/0x2d8 [nf_tables]
[ 72.172546] sp : ffff800081f2bce0
[ 72.172724] x29: ffff800081f2bd40 x28: ffff0000c2bb708c x27: 0000000000000038
[ 72.173078] x26: ffff0000c6780ef0 x25: ffff0000c643df00 x24: ffff0000c6778f78
[ 72.173431] x23: 000000000000001a x22: ffff0000c4b1f000 x21: ffff0000c6780f78
[ 72.173782] x20: ffff0000c2bb70dc x19: ffff0000c2bb7080 x18: 0000000000000000
[ 72.174135] x17: ffff0000c0a4e1c0 x16: 0000000000003000 x15: 0000ac26d173b978
[ 72.174485] x14: ffffffffffffffff x13: 0000000000000030 x12: ffff0000c6780ef0
[ 72.174841] x11: 0000000000000000 x10: ffff800081f2bcf8 x9 : ffff0000c3000000
[ 72.175193] x8 : 00000000000004be x7 : 0000000000000000 x6 : 0000000000000000
[ 72.175544] x5 : 0000000000000040 x4 : ffff0000c3000010 x3 : 0000000000000000
[ 72.175871] x2 : 0000000000003a98 x1 : ffff0000c2bb708c x0 : 0000000000000004
[ 72.176207] Call trace:
[ 72.176316] nft_rhash_gc+0x200/0x2d8 [nf_tables] (P)
[ 72.176653] process_one_work+0x178/0x3d0
[ 72.176831] worker_thread+0x200/0x3f0
[ 72.176995] kthread+0xe8/0xf8
[ 72.177130] ret_from_fork+0x10/0x20
[ 72.177289] Code: 54fff984 d503201f d2800080 91003261 (f820303f)
[ 72.177557] ---[ end trace 0000000000000000 ]---

Align struct nft_set_ext to word size to address this and
documentation it.

pahole reports that this increases the size of elements for rhash and
pipapo in 8 bytes on x86_64.