seccomp, bpf: disable preemption before calling into bpf prog

All BPF programs must be called with preemption disabled.

Fixes: 568f1967

 ("bpf: check that BPF programs run with preemption disabled")
Reported-by:  <syzbot+8bf19ee2aa580de7a2a7@syzkaller.appspotmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>