powerpc/svm: Don't issue ultracalls if !mem_encrypt_active()

Commit ad6c00283163 ("swiotlb: Free tbl memory in swiotlb_exit()")
introduced a set_memory_encrypted() call to swiotlb_exit() so that the
buffer pages are returned to an encrypted state prior to being freed.

Sachin reports that this leads to the following crash on a Power server:

[ 0.010799] software IO TLB: tearing down default memory pool
[ 0.010805] ------------[ cut here ]------------
[ 0.010808] kernel BUG at arch/powerpc/kernel/interrupt.c:98!

Nick spotted that this is because set_memory_encrypted() is issuing an
ultracall which doesn't exist for the processor, and should therefore
be gated by mem_encrypt_active() to mirror the x86 implementation.

Cc: Konrad Rzeszutek Wilk <[e-mail]>
Cc: Claire Chang <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Robin Murphy <[e-mail]>
Fixes: ad6c00283163 ("swiotlb: Free tbl memory in swiotlb_exit()")
Suggested-by: Nicholas Piggin <[e-mail]>
Reported-by: Sachin Sant <[e-mail]>
Tested-by: Sachin Sant <[e-mail]>
Tested-by: Nathan Chancellor <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]/
Signed-off-by: Will Deacon <[e-mail]>
Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>

arch/powerpc/platforms/pseries/svm.c

···
 
 int set_memory_encrypted(unsigned long addr, int numpages)
 {
+	if (!mem_encrypt_active())
+		return 0;
+
 	if (!PAGE_ALIGNED(addr))
 		return -EINVAL;
 
···
 
 int set_memory_decrypted(unsigned long addr, int numpages)
 {
+	if (!mem_encrypt_active())
+		return 0;
+
 	if (!PAGE_ALIGNED(addr))
 		return -EINVAL;