s390/bpf: Fix branch shortening during codegen pass

EMIT6_PCREL() macro assumes that the previous pass generated 6 bytes
of code, which is not the case if branch shortening took place. Fix by
using jit->prg, like all the other EMIT6_PCREL_*() macros.

Reported-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Fixes: 4e9b4a68 ("s390/bpf: Use relative long branches")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/net/bpf_jit_comp.c

@@ -248,8 +248,7 @@ static inline void reg_set_seen(struct bpf_jit *jit, u32 b1)
 
 #define EMIT6_PCREL(op1, op2, b1, b2, i, off, mask)		\
 ({								\
-	/* Branch instruction needs 6 bytes */			\
-	int rel = (addrs[(i) + (off) + 1] - (addrs[(i) + 1] - 6)) / 2;\
+	int rel = (addrs[(i) + (off) + 1] - jit->prg) / 2;	\
 	_EMIT6((op1) | reg(b1, b2) << 16 | (rel & 0xffff), (op2) | (mask));\
 	REG_SET_SEEN(b1);					\
 	REG_SET_SEEN(b2);					\