crypto: crct10dif-vpmsum - Fix missing preempt_disable()

In crct10dif_vpmsum() we call enable_kernel_altivec() without first
disabling preemption, which is not allowed.

It used to be sufficient just to call pagefault_disable(), because that
also disabled preemption. But the two were decoupled in commit 8222dbe2
("sched/preempt, mm/fault: Decouple preemption from the page fault
logic") in mid 2015.

The crct10dif-vpmsum code inherited this bug from the crc32c-vpmsum code
on which it was modelled.

So add the missing preempt_disable/enable(). We should also call
disable_kernel_fp(), although it does nothing by default, there is a
debug switch to make it active and all enables should be paired with
disables.

Fixes: b01df1c1 ("crypto: powerpc - Add CRC-T10DIF acceleration")
Acked-by: NDaniel Axtens <dja@axtens.net>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

arch/powerpc/crypto/crct10dif-vpmsum_glue.c

@@ -44,10 +44,13 @@ static u16 crct10dif_vpmsum(u16 crci, unsigned char const *p, size_t len)
 
 	if (len & ~VMX_ALIGN_MASK) {
 		crc <<= 16;
+		preempt_disable();
 		pagefault_disable();
 		enable_kernel_altivec();
 		crc = __crct10dif_vpmsum(crc, p, len & ~VMX_ALIGN_MASK);
+		disable_kernel_altivec();
 		pagefault_enable();
+		preempt_enable();
 		crc >>= 16;
 	}