bpf: Support narrow loads from bpf_sock_addr.user_port

bpf: Support narrow loads from bpf_sock_addr.user_port

bpf_sock_addr.user_port supports only 4-byte load and it leads to ugly
code in BPF programs, like:

	volatile __u32 user_port = ctx->user_port;
	__u16 port = bpf_ntohs(user_port);

Since otherwise clang may optimize the load to be 2-byte and it's
rejected by verifier.

Add support for 1- and 2-byte loads same way as it's supported for other
fields in bpf_sock_addr like user_ip4, msg_src_ip4, etc.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/c1e983f4c17573032601d0b2b1f9d1274f24bc16.1589420814.git.rdna@fb.com