octeontx2-af: avoid off-by-one read from userspace

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

octeontx2-af: avoid off-by-one read from userspace

We try to access count + 1 byte from userspace with memdup_user(buffer,

count + 1). However, the userspace only provides buffer of count bytes and

only these count bytes are verified to be okay to access. To ensure the

copied buffer is NUL terminated, we use memdup_user_nul instead.