bpf: fix build error with clang

Building the newly introduced BPF_PROG_TYPE_SK_REUSEPORT leads to
a compile time error when building with clang:
net/core/filter.o: In function `sk_reuseport_convert_ctx_access':
  ../net/core/filter.c:7284: undefined reference to `__compiletime_assert_7284'

It seems that clang has issues resolving hweight_long at compile
time. Since SK_FL_PROTO_MASK is a constant, we can use the interface
for known constant arguments which works fine with clang.

Fixes: 2dbb9b9e6d ("bpf: Introduce BPF_PROG_TYPE_SK_REUSEPORT")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/core/filter.c

@@ -7281,7 +7281,7 @@ static u32 sk_reuseport_convert_ctx_access(enum bpf_access_type type,
 		break;
 
 	case offsetof(struct sk_reuseport_md, ip_protocol):
-		BUILD_BUG_ON(hweight_long(SK_FL_PROTO_MASK) != BITS_PER_BYTE);
+		BUILD_BUG_ON(HWEIGHT32(SK_FL_PROTO_MASK) != BITS_PER_BYTE);
 		SK_REUSEPORT_LOAD_SK_FIELD_SIZE_OFF(__sk_flags_offset,
 						    BPF_W, 0);
 		*insn++ = BPF_ALU32_IMM(BPF_AND, si->dst_reg, SK_FL_PROTO_MASK);