Most of the code in bpf_setsockopt(SOL_SOCKET) are duplicated from
the sk_setsockopt().  The number of supported optnames are
increasing ever and so as the duplicated code.

One issue in reusing sk_setsockopt() is that the bpf prog
has already acquired the sk lock.  This patch adds a
has_current_bpf_ctx() to tell if the sk_setsockopt() is called from
a bpf prog.  The bpf prog calling bpf_setsockopt() is either running
in_task() or in_serving_softirq().  Both cases have the current->bpf_ctx
initialized.  Thus, the has_current_bpf_ctx() only needs to
test !!current->bpf_ctx.

This patch also adds sockopt_{lock,release}_sock() helpers
for sk_setsockopt() to use.  These helpers will test
has_current_bpf_ctx() before acquiring/releasing the lock.  They are
in EXPORT_SYMBOL for the ipv6 module to use in a latter patch.

Note on the change in sock_setbindtodevice().  sockopt_lock_sock()
is done in sock_setbindtodevice() instead of doing the lock_sock
in sock_bindtoindex(..., lock_sk = true).
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/r/20220817061717.4175589-1-kafai@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>