target/ppc/kvm: Avoid using alloca()

kvmppc_load_htab_chunk() is used for migration, thus is not
a hot path. Use the heap instead of the stack, removing the
alloca() call.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20250901132626.28639-2-philmd@linaro.org>

target/ppc/kvm.c

@@ -2760,11 +2760,11 @@ int kvmppc_save_htab(QEMUFile *f, int fd, size_t bufsize, int64_t max_ns)
 int kvmppc_load_htab_chunk(QEMUFile *f, int fd, uint32_t index,
                            uint16_t n_valid, uint16_t n_invalid, Error **errp)
 {
-    struct kvm_get_htab_header *buf;
-    size_t chunksize = sizeof(*buf) + n_valid * HASH_PTE_SIZE_64;
+    size_t chunksize = sizeof(struct kvm_get_htab_header)
+                       + n_valid * HASH_PTE_SIZE_64;
+    g_autofree struct kvm_get_htab_header *buf = g_malloc(chunksize);
     ssize_t rc;
 
-    buf = alloca(chunksize);
     buf->index = index;
     buf->n_valid = n_valid;
     buf->n_invalid = n_invalid;