bpf: Fix SO_RCVBUF/SO_SNDBUF handling in _bpf_setsockopt().

The commit 4057765f ("sock: consistent handling of extreme
SO_SNDBUF/SO_RCVBUF values") added a change to prevent underflow
in setsockopt() around SO_SNDBUF/SO_RCVBUF.

This patch adds the same change to _bpf_setsockopt().

Fixes: 4057765f ("sock: consistent handling of extreme SO_SNDBUF/SO_RCVBUF values")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220104013153.97906-2-kuniyu@amazon.co.jp