x86: bpf_jit_comp: support BPF_S_ANC_SECCOMP_LD_W

This patch implements the seccomp BPF_S_ANC_SECCOMP_LD_W instruction
in x86 JIT, by simply calling seccomp_bpf_load().

SEEN_SKBREF was suggested by Eric Dumazet.  SEEN_SKBREF shouldn't be
set in seccomp filters.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Will Drewry <wad@chromium.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: David Laight <david.laight@aculab.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Nicolas Schichan <nschichan@freebox.fr>
x86: bpf_jit_comp: optimize BPF_S_ANC_SECCOMP_LD_W

This patch further optimizes JIT for seccomp filters.  It removes the
call to seccomp_bpf_load() and directly emits instructions instead.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Will Drewry <wad@chromium.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: David Laight <david.laight@aculab.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Nicolas Schichan <nschichan@freebox.fr>