libbpf: add BPF_CALL_REL() macro implementation

Add BPF_CALL_REL() macro implementation into include/linux/filter.h
header, which is now used by libbpf code for feature detection.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

include/linux/filter.h

@@ -37,6 +37,13 @@
 		.off   = 0,					\
 		.imm   = IMM })
 
+#define BPF_CALL_REL(DST)					\
+	((struct bpf_insn) {					\
+		.dst_reg = 0,					\
+		.src_reg = BPF_PSEUDO_CALL,			\
+		.off = 0,					\
+		.imm = DST })
+
 #define BPF_EXIT_INSN()						\
 	((struct bpf_insn) {					\
 		.code  = BPF_JMP | BPF_EXIT,			\