bpf: Bump iter seq size to support BTF representation of large data structures

BPF iter size is limited to PAGE_SIZE; if we wish to display BTF-based
representations of larger kernel data structures such as task_struct,
this will be insufficient.
Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/1601292670-1616-6-git-send-email-alan.maguire@oracle.com

kernel/bpf/bpf_iter.c

@@ -88,8 +88,8 @@ static ssize_t bpf_seq_read(struct file *file, char __user *buf, size_t size,
 	mutex_lock(&seq->lock);
 
 	if (!seq->buf) {
-		seq->size = PAGE_SIZE;
-		seq->buf = kmalloc(seq->size, GFP_KERNEL);
+		seq->size = PAGE_SIZE << 3;
+		seq->buf = kvmalloc(seq->size, GFP_KERNEL);
 		if (!seq->buf) {
 			err = -ENOMEM;
 			goto done;