bpf: Change bpf_getsockopt(SOL_SOCKET) to reuse sk_getsockopt()

bpf: Change bpf_getsockopt(SOL_SOCKET) to reuse sk_getsockopt()

This patch changes bpf_getsockopt(SOL_SOCKET) to reuse
sk_getsockopt().  It removes all duplicated code from
bpf_getsockopt(SOL_SOCKET).

Before this patch, there were some optnames available to
bpf_setsockopt(SOL_SOCKET) but missing in bpf_getsockopt(SOL_SOCKET).
It surprises users from time to time.  For example, SO_REUSEADDR,
SO_KEEPALIVE, SO_RCVLOWAT, and SO_MAX_PACING_RATE.  This patch
automatically closes this gap without duplicating more code.
The only exception is SO_BINDTODEVICE because it needs to acquire a
blocking lock.  Thus, SO_BINDTODEVICE is not supported.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20220902002912.2894040-1-kafai@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>