apparmor: Replace deprecated strcpy in d_namespace_path

apparmor: Replace deprecated strcpy in d_namespace_path

strcpy() is deprecated; replace it with a direct '/' assignment. The
buffer is already NUL-terminated, so there is no need to copy an
additional NUL terminator as strcpy() did.

Update the comment and add the local variable 'is_root' for clarity.

Closes: https://github.com/KSPP/linux/issues/88
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: John Johansen <john.johansen@canonical.com>