bpf: fix restrict_fs on s390x

bpf: fix restrict_fs on s390x

Linux kernel's bpf-next contains BPF LSM support for s390x. systemd's
test-bpf-lsm currently fails with this kernel.

This is an endianness issue: in the restrict_fs bpf program,
magic_number has type unsigned long (64 bits on s390x), but magic_map
keys are uint32_t (32 bits). Accessing magic_map using 64-bit keys may
work by accident on little-endian systems, but fails hard on big-endian
ones.

Fix by casting magic_number to uint32_t.