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>