bpf: export crash_kexec() as destructive kfunc

Allow properly marked bpf programs to call crash_kexec().

Signed-off-by: Artem Savkov <asavkov@redhat.com>
Link: https://lore.kernel.org/r/20220810065905.475418-3-asavkov@redhat.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>