s390/crypto: use vector instructions only if available for ChaCha20

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

s390/crypto: use vector instructions only if available for ChaCha20

Commit 349d03ffd5f6 ("crypto: s390 - add crypto library interface for
ChaCha20") added a library interface to the s390 specific ChaCha20
implementation. However no check was added to verify if the required
facilities are installed before branching into the assembler code.

If compiled into the kernel, this will lead to the following crash,
if vector instructions are not available:

data exception: 0007 ilc:3 [#1] SMP
Modules linked in:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.3.0-rc7+ #11
Hardware name: IBM 3931 A01 704 (KVM/Linux)
Krnl PSW : 0704e00180000000 000000001857277a (chacha20_vx+0x32/0x818)
R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
Krnl GPRS: 0000037f0000000a ffffffffffffff60 000000008184b000 0000000019f5c8e6
0000000000000109 0000037fffb13c58 0000037fffb13c78 0000000019bb1780
0000037fffb13c58 0000000019f5c8e6 000000008184b000 0000000000000109
00000000802d8000 0000000000000109 0000000018571ebc 0000037fffb13718
Krnl Code: 000000001857276a: c07000b1f80b larl %r7,0000000019bb1780
0000000018572770: a708000a lhi %r0,10
#0000000018572774: e78950000c36 vlm %v24,%v25,0(%r5),0
>000000001857277a: e7a060000806 vl %v26,0(%r6),0
0000000018572780: e7bf70004c36 vlm %v27,%v31,0(%r7),4
0000000018572786: e70b00000456 vlr %v0,%v27
000000001857278c: e71800000456 vlr %v1,%v24
0000000018572792: e74b00000456 vlr %v4,%v27
Call Trace:
[<000000001857277a>] chacha20_vx+0x32/0x818
Last Breaking-Event-Address:
[<0000000018571eb6>] chacha20_crypt_s390.constprop.0+0x6e/0xd8
---[ end trace 0000000000000000 ]---
Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

Fix this by adding a missing MACHINE_HAS_VX check.

[agordeev@linux.ibm.com: remove duplicates in commit message]