selftests/bpf: Fix progs/test_deny_namespace.c issues.

The following build error can be seen:
progs/test_deny_namespace.c:22:19: error: call to undeclared function 'BIT_LL'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        __u64 cap_mask = BIT_LL(CAP_SYS_ADMIN);

The struct kernel_cap_struct no longer exists in the kernel as well.
Adjust bpf prog to fix both issues.

Fixes: f122a08b ("capability: just use a 'u64' instead of a 'u32[2]' array")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>