bna: ensure the copied buf is NUL terminated

bna: ensure the copied buf is NUL terminated

Currently, we allocate a nbytes-sized kernel buffer and copy nbytes from
userspace to that buffer. Later, we use sscanf on this buffer but we don't
ensure that the string is terminated inside the buffer, this can lead to
OOB read when using sscanf. Fix this issue by using memdup_user_nul
instead of memdup_user.

Fixes: 7afc5dbde091 ("bna: Add debugfs interface.")
Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Link: https://lore.kernel.org/r/20240424-fix-oob-read-v2-2-f1f1b53a10f4@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>