bpf/syscall: allow key to be null in map functions
This commit adds the required logic to allow key being NULL
in case the key_size of the map is 0.
A new __bpf_copy_key function helper only copies the key from
userpsace when key_size != 0, otherwise it enforces that key must be
null.
Signed-off-by: Mauricio Vasquez B <mauricio.vasquez@polito.it> Acked-by: Song Liu <songliubraving@fb.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org>